Adoption Support Group

Hi
I am trying to sort some things out in my mind and I could use some advise. We were contacted by a woman that said she was my husbands sister. She gave us all the infor. and it all fell into place. My husband asked his mom and she said yes. She tellls two storys. To my husband she said she was raped. On the adoption paper work it says talks about her and her boyfriend. She was 17 years old at the time. The paper work nor the birth certificate gives the dads name.
Over time we have gotten to know my sister in law. My mother in law will not talk about her or meet her. She wants nothing to do with her. We want a relationship with my sister in law. The problem my mother in law lives very close to us. (5 minutes) We are the only ones around to take care of her. She is 70 years old. So we are trying to figure out how we will handle the holidays and times when my sister in law comes to visit.
My sister in laws parents have died and she now has no one beside us. I always though ok we are protecting my mother in law. We did not tell the family.
I do not want to hurt my mother in law but this is really laying heavy on my mind. My sister in law did nothing wrong to my mother in law. Even if she was raped that was 54 years ago. This woman needs her mom. My mother in law is very much a bull head woman who feels what she says goes. Anyone have any suggestions on anything I could read or say to my mother in law. I feel really lost and want to help my sister in law.
Sorry for going on and on. It just seems like I am protecting the wrong person.
Elizabeth