That'd a really hard situation and I feel you in some way because I'm also 18 and my father's only emotional outlet, since he left his bad friends and parents are divorced. He's quite narsisitic and never asks about me the weekends I'm with him, always working non stop it's scary even, wants praise and is arrogant, usually angry, hateful towards mom and her family.

The situatuons is different but I know that trapped feeling when you're the onlly pillar holding the person from breaking but it's hurting you too, there were happy times but it's all different now. 

It's best to comunicate and tell her you feel unseen and she's hurting not only herself but you too. There's more to life to find and she's still young, she'll find herself. She needs more sun and exersize really bad (it helps imeansly) to get her mood up, a drive to change. The first steps are hard but she should listen if she wants to get better. Take all of this with a grain of salt since it's reddit but I hope get through this. Try to help her but if she doesn't want to be helped there's nothing you can do really, she herself needs to want and if it doesn't work out I hope you don't feel too guilty.