term of endearment: a word or phrase used to address and/or describe a person, animal, or inanimate object for which the speaker feels love or affection.
Justin never calls Alex 'honey'. Nor 'dear', 'sweetheart', 'sweetie', 'love'.
He calls her by her name: Alex. Or sometimes, special times, Alexandra. Very rarely, he'll call her by her full name: Alexandra Margarita Russo.
It's not that he doesn't understand the concept - ask him what a term of endearment is, and he can tell you just what it says above. Ask him for examples, and he could produce all those words, and many more besides.
But he won't call Alex any of them.
It's not that he doesn't love her - don't think that for a second. Justin lives for Alex. He'd die for her, kill for her, do anything that was needed. No, that's not it at all.
The fact of the matter is this: Justin Russo is nothing if not precise in his speech. 'Honey', 'sweet', 'sugar' and all their variants are foods and tastes, and while, if cornered, he would admit that he likes the taste of Alex, she's not food.
'Dear' is too generic - it's a salutation, he'd say, not a term of affection. You can start a letter 'dear sir or madam', and a word you can use for a generic stranger is not one that can possibly describe how he feels about Alex.
'Love' is, perhaps, the best - but he loves his mom and dad, and his brother Max. He loves comics, and hot tea, and quiet. It's too broad a word, unmodified. He could retreat into Greek, but to say philos, eros, and agape each time he addresses her would be too much - for he feels all of those for her.
And so we come back to her name: Alex. Since Justin Russo first learned to speak, it's been the word he's used for the one person who constantly confuses and delights him. The center of his world, the one he holds above all others in his thoughts, in his feelings, in his heart.
He speaks half a dozen languages, but in all of them, there is no word that carries even the merest shadow of the feelings that his sister's name arouses in him - and thus, he calls her by her name, because that is the greatest term of endearment he has ever known, or will ever know.
Author's note: I was reading a fic a few days ago in which Justin called Alex 'honey', and it just struck me as wrong... and then I realized that I don't believe I've ever had Justin use any terms of endearment for Alex in any of the stories I've written so far. Now, I know why that is... but I thought it might be good to share it.