Context of this story: Robb Stark never met Jeyne/Talisa and intends to keep his oath. Therefore the Freys never had any reason to take revenge and ally themselves with the Lannisters, in hope of having a Frey daughter crowned. The Boltons remained loyal as well.
Theon Greyjoy took Winterfell and "killed" the Stark boys. Ramsay Snow is never sent to retake Winterfell.
Renly Baratheon is dead and his brother Stannis died at the battle of the Blackwater.
Margaery and Joffrey are bethroted and inted to wed after the war.
You could place this somewhere in season 4.

« … And Sansa Stark is set to be married to Jaime Lannister as soon as he comes back to Kingslanding. »

The tent went silent. They all stared at the King in the North. His facial expression revealed nothing. The lords watched Catelyn Stark. It was very clear she was about to break down.

"How certain is this?" asked Robb Stark.

"My spies don't lie, Your Grace. And Lord Karstark's spies confirm it. It's not a secret in King's Landing." said Roose Bolton quietly, afraid of Robb Stark's reaction.

The King in the North was silent, staring ahead.

Then suddenly, he rushed out of the tent as Catelyn Stark started to lose control.

Robb didn't walk. He didn't run either. Where would he go anyway?

How could he do that? Is that Tywin Lannister truly a heartless gold-shitting monster? Robb rushed back into the tent. He was furious. Although he barely noticed it, he could see it on the faces of the other lords.

"How does he intend to marry his son to my sister if his son is our captive?" he screamed.

The lords didn't answer, tried not to look at the young wolf in the eyes.

"How?"

Lord Raymond, young and brave, answered. "He probably has a plan, Your Grace. A plan we don't know just yet. But he's Tywin Lannister; he wouldn't even think about this if it won't work."

Robb took a step closer to Lord Raymond. "It won't work. I will stop this."

He took a quick look at his mother, who was crying softly, not even trying to hide her tears like a lady, before adding: "I want to know more about this. Quickly. Make it our top priority."

"Put it before Harrenhall matters? And Theon Greyjoy's? And before your sister Arya and your brothers…"

"Are they close to be found?"

"No."

"Then yes. Sansa is the only one we know for sure is alive. I won't let her slip into Lannister hands. She's our number one priority."

The lords nodded, and the King in the North got out of the tent as soon as he could. He couldn't bare to comfort his mother; she would blame him for this. Blame him for not returning the Kingslayer soon enough. He felt guilty enough already. And he was about to show his anger to the Kingslayer.

He entered the cage, surprising Jaime Lannister, followed by Grey Wind.

"Your Grace" said Jaime Lannister mockingly, "What a pleasure to see you here in my humble home."

"Shut up, Kingslayer," said Robb Stark furious.

Jaime Lannister was surprised. What could have angered the young wolf so much that he had to come here and threaten him with his huge direwolf?

"What do you want?" Jaime asked.

"I want your head. And your father's. And every single man's head that ever thought of hurting my sister."

"Your sister?" Jaime asked confused. "You found the little girl? Arya Stark?"

"Not her."

"Sansa?"

"Who else, Kingslayer?"

"I have no idea what you want from me. News? Well, let me think... Last time I was there, she… was in the hands of her loving husband-to-be."

"You."

"What?"

"Don't pretend, Kingslayer. Just…" Grey Wind approached him. "Tell me why you are engaged to my sister. Or my wolf will lose his temper, and so will I."

"Engaged? I have no idea what you're talking about. Sansa Stark is to be Joffrey's queen as soon as she bleeds."

"Or your wife. Stop lying, Kingslayer. Your father must have thought about this plan carefully. You know him. Tell me why…" Grey Wind showed his teeth. "And I may not have Grey Wind leave a scar on your pretty little Lannister face. Your sister wouldn't like that either, right? Think of her before you say something stupid."

Jaime Lannister had no idea what to do.

"You really think I know anything of this?! When I left, she was happily engaged to Joffrey!"

"You know your father. Explain to me…" He started to talk louder and angrier. "… why he would do that!"

He knew. Of course Robb knew. He just couldn't bare to say the words. To say what everyone knew.

Jaime stayed silent.

"You think that not saying anything is going to save your pretty face?! I hear enough words of Joffrey hurting my sister! Humiliating her, beating and stripping her in front of everyone! I'm not far from doing much worse to you! It's only what you deserve!" He meant it. It was very clear. And his wolf seemed to think the same way as he.

"If this is true…" started Jaime. "It's to secure the North should you fall."

Robb didn't seem to think that was enough. He wanted more words, so this would feel real.

"A Lannister son, born by your Stark sister, would rule the North."

Robb was furious. "That will NEVER happen!" The whole camp must have heard him. He noticed how the only noise was Grey Wind's heavy breathing.

"My sister would NEVER bear your children."

"How can she if I'm here sitting in my own shit?" The Kingslayer was back.

"Shut up. You wouldn't mind marrying her."

"She's just a child!"

"She won't be if you marry her."

"I don't want her."

"No, you want your incestuous sister."

Jaime ignored that remark. "She's just a girl."

"A pretty, young Stark girl. What more could anyone want?"

"Yes, she's pretty. And I'm sure she'll grow very beautiful. But I might be the only man in the Seven Kingdoms who wouldn't want her." He wanted Cersei.

"And you would dare to say no to your father if he told you to marry her?"

That was a smart comment. No-one dared to say no to his rich, Lannister father.

After a minute of silence, Robb started to calm down, but his fury didn't. "You will never marry my sister. I can assure you… Every single Northerner alive will make sure that doesn't happen."

"Oh, that's not hard to believe. The Northerners protect the Starks. They don't obey by fear, but by love. I've always admired that," said Jaime, trying to sooth Robb Stark with his words.

"Don't try to change the subject. We will send a raven to Kingslanding… Every scar left on her, will be doubled on you." He ment it. Clearly.

"I'm sure my nephew takes good care of her and everything you hear are just rumors."

"I'm sure your son is not the honorable kind of man. Everyone who has been in Kingslanding knows it. They pity her. But not one of them will stand up for her."

"They value their head more than their honor. In the end, everyone does."

Robb was silent. Jaime stared into his cold, blue, sad eyes. "Not everyone."

Sansa stared through the window. The moon was shining with all the pretty stars around it. She had always loved that sight, but she couldn't admire now.

Jaime Lannister. Really? Is that the best the Lannisters could do? Probably. It was clever, really. A Lannister heir to the Northern seat. It was so cunning it made her sick.

Robb wasn't like that. Robb was truthful, dutiful and honorable. Nothing like those Lannisters. Nothing like anyone in Kingslanding, really. He was like father.

But wouldn't father have come and save her by now? It had been over a year. Over a year since she had seen Winterfell, her family. Over a year since she had seen snow. Strangely enough, she missed it so badly. When she was there, the only thing she could think about was the South. She hated the snow and the cold winds. But now that she was in the poisonous South, the North seemed so peaceful and innocent. What if she never saw snow again? That made her heart race. She couldn't bare that, for sure. But strangely, she had the feeling she would see snow again, in time. After all, Winter is Coming. And she had survived so far.

"Robb…" They were in his private tent. His mother wanted comfort. She wanted her tall and strong son to hold her in her arms. To tell her everything was going to be okay. But he turned away. "Robb? My dear boy. Come hug me." Her voice trembelled.

He turned to her. She saw the guilt in his eyes, the pain. "Don't…"

"It's my fault. I should have traded the Kingslayer. I should have gone and save Sansa, I should have kissed Joffrey's feat just to get her out of there!"

"No, Robb. You're fighting for a cause. For the North." He knew she was just saying the things he wanted to hear. All she cared about was family, duty, honor. Family most of all. She would have traded every single Northerner if that would have got her her girls back. She blamed him. She blamed him for choosing the North over his family. Over Sansa.

"You're lying. You know I'm right. You blame me."

She couldn't find the strength to deny it.

"You think I don't love her. But I do, mother, I do. I love my sister. I love her dancing hair when she runs. I love her voice when she sings. I love her when she tells her tales of knights and maidens fair. I love her when she plays with Lady. I love her naivety, her idealism, her faith. I want to see her grow more and more beautiful every day. I want to see her turn into a mature woman, I want to be there for her to protect her against those Kingslanding men. I even want to see her naivety crushed; that's how badly I miss her. I want to see her grow stronger every day. I knew all along she was a Stark. Everyone thought she was a Tully: wrong. Sansa has always been a Stark. And I'm sure she will survive whatever winter that comes to her. Because she's a Stark, in her heart and soul."

"Robb…" She started to cry softly.

"But you know what I miss the most, mother?"

She closed her eyes, preparing for something to break her.

"Her laugh. Her giggly, innocent and light laugh. It sounds so good. Even to Arya, even though she'd deny it."

Catelyn cried. She cried and cried and cried for the only daughter left to her, that she was so scared she'd loose.

Robb held her. Not for long, but long enough to give her courage and strength.

"I'll get her back, mother. I promise. I will get my sister back."

Sansa sighed. She couldn't sleep. Her pillow was wet and sticky, but she ignored it. She had felt that so many times. She thought of her time in Winterfell. When she hugged Rickon. When she played with Bran. When she fought with Arya. When she laughed with Jon. When she listened to her father's stories and when her mother brushed her hair. And then she thought about Robb. She could barely believe it when he was named the King in the North. Robb? Robb, her brother, barely taller than her? She couldn't imagine how tall he'd be now. Just like father.

She remembered when he took her on his back and walked her around Winterfell. It seemed so silly to others, but so fun to them. They were little. But old enough for Robb to already feel protective. He had pushed her in the snow, and sat next to her. He had told her that until her tall and gentle and strong knight would come, he would be there. He would be there to protect her from the mean monsters. He had kissed her on her forehead. And she had smiled. But then, she had ran away because she had to pee badly.
Sansa laughed. A lady wouldn't do that. But she had done it. And for once, she didn't care. Arya would be proud of her for that.

Sansa felt her brother's comfort, even though he was far away from her. But Robb had broken his promise. He hadn't been there for her to protect her from Joffrey, the Queen and ser Meryn and Illyn Payne. And, as hard as it was, Sansa realized she couldn't count on him anymore. She had to fight her own court battles, not counting on her brother to come and save her. She had to put up her Wall of Ice, so high no one could breach it. She had to be as brave as a Stark of Winterfell. Because she was lady Sansa, with a skin that had turned from porcelain, to ivory, to steel. She would outlive them all.