My mother-in-law cannot stay here anymore, I can't deal with it. What can I do?

My mother in law moved in with us 3 years ago, for the 7 years before that my husband lived with her, to help her financially and because she couldn't be by herself for a peroid longer than a few hours. She would have mini strokes. She did not prepare for her retirement, my husbands father walked out on her 20 years ago, because she was selfish . My husband and I have 3 grown children that she didn't really achknowledge when they was growing up. She also has another granddaughter by her other son, that all she did for that girl was praise her for everything she did, and gave and gave to her. Her other son is a Alcholoic so he is no help at all. Infact he has told her, he would rather not have anything to do with her ever again for some of the things she did to him. My hisbands sister is diseased from a Drug over dose. Anyway, 3 years ago, I said she needed to move here with us, I wasn't going to be without my husband anymore for his mom. So she moved in, she gets $749.00 per month, out of that she has to pay 189.00 per month for her supplemental insurance to her medicare. That leaves about 500.00 a month. She doesn't drive, she does nothing but sit. She is 75 years old. Her memory is going. About a year ago, my daughter came to me (Which she is 30 now) and asked me why we never did anything about her being molested when she was younger. Which took my husband and I by surprise we never knew. My daughter than told us that she had told her grandmother when it happened (My daughter was 9 at the time) . My husbands mother never said a word to us, and she told my daughter that, it was ok, she had been molested as well when she was little. The person that did this was my mother in laws nephew and this went on until my daughter was 12. Always when my daughter would go with her Grandmother to visit relatives in another city. So I confronted my mother in law about it, she said she didn't remember that. I then contacted another member of the family because my daughter had said she even called this other family member to come and get her on numerous occassions and they did, but my daughter never told them why she needed to be picked up. I asked my daughter why she didn't tell us and she said she told grandma and grandma said it was ok. So I was and still is pissed. Now I am at a point she has been here almost 3 years, has no money, no family left and I want her gone. She plays the poor pitiful me card, and she actually feels everyone owes her. She thinks her son should take care of her until the day she dies. I can't and will not continue this. It just kills me everytime I look at her, and with me knowing that she could have helped my daughter and choose not to, and yes this is her biological granddaughter. The granddaughter that she praised growing up hasn't spoken to her in 2 years. So I guess my question is, where can she go with no money except for Social Secuirty? She can't stay here anymore, I can't deal with it. My husband can't physically take care of her, he can't take care of himself, 3 back surgeries later due to his profession before he retired, So I am left with it all. The driving, the cleaning, the cooking EVERYTHING. My husband says he will support me with any thing that I decide to do. His mother was a only child with everything given to her, and this is what is left. Am I wrong to feel it is not my place to have to take care of her ? I need soe support and some suggestions, my 3 children tolerate her when they come over to see us, but want nothing to do with helping with her at all. What she choose not to do to help our daughter effected our other 2 children and the way they feel about her. What to do ???????????