I believe the "fourths legacy" was refering to the sealed Kyuubi inside of Naruto.
When they say the "Fourth's Legacy," Itachi merely means that Naruto was left behind by the Fourth as an incarnation of Kyuubi. Doesn't necessarily mean they're related in any way.
i never really thought about that, i guess.

I think that Sasuke acts the way he does is because if people get close to him, they would just hold him down and get in his way, therefore he just pushes everyone away.
Sasuke probably doesn't want to think about his family because it reminds him of how useless and unpowerful he was back then