How did I get myself into this?

I kept telling myself that I just needed a little time to adjust to the situation at home when my mother-in-law had to move in with us temporarily. Unfortunately, her plans fell through and temporary became permanent. I've tried so hard to be selfless and keep my mouth shut, but as time goes by, I find myself feeling bitter and resentful. I hate that feeling.
The last few days have been really hard on me and I feel like I've reached my limit and I have no idea what to do. On one hand she is Rory's mom and I know he worries about where she would go or what she would do if she wasn't with us. On the other hand, the drama and the stress is taking it's toll on our whole family.
For any of you that don't know her, she is a very different type of person. She is beyond a handful and it gets worse as the time goes on. Well, she talked her doctor into prescribing a diet pill for her. (It's been explained that it has a similar effect as speed). She has been taking this drug for a little while now and has been a bit more jittery and getting less sleep. Recently there was a situation that happened with her other son Derek that she got involved in and it "charged" her emotionally. With all those factors, she was bound to crack, and she did. She was practically hysterical when she was trying to explain that God was talking to her and that all of our worries were over. Now, I do believe in revelation and inspiration, but this woman was three shades of crazy and had no clue what she was talking about. She said that God had told her that some stranger was going to pay off our house so I could be a stay at home mom and Rory could be a stay at home dad and that she was going to find her "eternal mate" and get married and travel all over the world and bring millions of people back to God. In the middle of her tirade, she lost the ability to speak several times and once she recovered, she said it was Satan holding her tongue and that God had pushed him away to enable her to speak again.
The worst part of this entire situation was that, as I was trying to calm her down, there was a knock at my door. When I answered it, there was a lady I hadn't met before standing there with a little girl. To my shock and horror, I noticed that the little girl was mine! Lexie had wandered out the front door that had been left unlocked or possibly open by one of Cassies other family members bringing her back to my house. Needless to say my heart instantly sunk into my stomach and I started bawling my eyes out. What kind of mother am I, that instead of being able to protect my daughter and keep her safe I was trying to console a crazy lady that had all but turned our family upside down?! All the "what if's" started spinning through my head. What on earth would I do If anything ever happened to my little girl. I am literally at my breaking point. I am afraid that this was the very last straw. I am thinking that it has come down to she goes or I go.