AlcSis.....I am going to tell this to you, very straight forward----I think to let her come live with you is the very worst thing you can do.
Basing this statement on the information that you have posted. If she needs help with housing (I haven't a clue to where she lives or her age)...there are social services organizations that can help her with affordable housing.
It sounds like you are intimidated---and, are afraid to look like the "bad guy". You will have to get honest enough to tell her the truth---that you predict too much conflict and that your health status, unfortunately, does not allow for it. Period.
If you "chicken out"....you may go from "nice guy" status to "abused guy" status......
I really do feel for you.....but, there are times when you just have to call a spade a "spade"......
By the way......WELCOME TO THE FORUM!!!!!!!!!
DANDYLION