Actually, Littlefoot and Ali aren't related by blood at all. They're only the same species, and that's about it. The only reason Grandpa called them "cousins" is because there generally weren't many longnecks in the valley at the time. I know this because if they were in fact related by blood, then they would've certainly greeted each other in a different way, or recognised each other. So you don't need to worry about any of that "cousins" stuff.