Honestly, the best thing you can do is break all contact with her. The person you knew is dead and you should act as such. Grieve your loss and move on with your life.

I went through a few emotionally abusive relationships and for your sake, please let it go. You're already showing signs of having been abused by this relationship and the longer you stay the worse it will get. Remember the good times you did have, but she is gone now and you won't ever get her back if you just let yourself be the punching bag. Your cousin is a zombie.. it looks like her but she's not there anymore.

I don't think you should entirely lose hope. Though your cousin as you remember her is gone, there is a chance that she'll see what he's done and try to change. She might even become a decent person again, but it'll never really be who you remember. Now, listen very carefully to this next part because you might be thinking that you can help her change and that you'll just deal with the abuse until then..

You can not help her while you remain close to her. Nothing you say or do will convince her to change. Instead, the more you try and fight what she is becoming, the more you will drive her into it. This is why cutting contact is so important. If anything of who she was is left, the loss of someone so close will hit her and maybe even help her reflect on what she's becoming.

That being said, do not let her back into your life for at least a year after the day you start no contact. Do not do it.. She'll know then that you'll never follow through and she can do to you whatever abusive thing she wants. It'll be even harder to leave again if you let her back into your life. Save yourself and you may save her future.. stick with it and you'll only suffer and help no one.