Yes, she is taking advantage of you! You and your family and it seems everyone else she comes in contact with. Please get her out of your life and tell your mother to stop letting her back in. That's not right, whether it's your parents' house or not, for that to keep happening. Maybe you need a place of your own so you can control who you're forced to live with. She sounds crazy to me, this woman. I don't believe her about the baby, I suspect she made that up to get you to let her back into your life, and the whole thing with getting your mother to let her live with you - ugh! Forgot where the grave is? Nope, that doesn't wash.
You are being used. Please take back your life, for your own sake. There's some really nice gal out there for whom you'd make such a great boyfriend or spouse. One who will love you and want a normal relationship instead of this - whatever it is, it's not love. Don't waste another minute on this one.
Let her know in no uncertain terms that she's out of your life, and if she keeps bothering you or tries to move in again call the police. You live there, and you have a right to not have her in your life.