Kristen is the sister who seems most unattached to the rest of the family. She's off in the big city living her own life and trying to redefine herself—at the beginning of the book she's the only Greene sister with the guts to proclaim that modeling just isn't for her. In some ways she seems removed from all the family problems that are going on and practically washes her hands of Whitney's problems:
At first Kristen was hurt, then angry, before finally retaliating with her own silence. (3.145)
But as time goes on, it becomes clear that Kristen isn't completely separate from her family. She may be trying to forge her own life, but she's still intrinsically connected to the people that she loves the most. Whitney's rejection of her hurts her feelings, even if she won't admit it. And when she takes a film class, she ends up making a deeply personal film about her relationship with Whitney.
Kristen may live in a different city and have a different lifestyle from the rest of her family, but she's still connected to them. And in the end she comes home so that they are all together again for the holidays, and Whitney and Annabel are both able to open up about their problems in front of her. Though she might not always be physically present, she is still part of the ever-so-important support system that allows them each to heal their wounds.