Elderly Mom living with mentally-ill grown daughter.

I have an 84 yr. old mother. After my dad passed last year she made a new will. Informed me that my mentally ill sister who is 40 will inherit her home and take care of her. My sister is bi-polar and former drug addict, and has been in jail due to a psychotic episode with the authorities. She is on disability. She cannot handle finances. I live in a different state. My only way of communication is by phone, or drive 9 hours up to check on them. My sister has forbidden me to come visit or stay at my mother's home. Every time I call, send gifts etc., I am told not to bother, not to do anything to help. They have every thing under control. I sent a nice Easter cake and was yelled at over the phone not to do that ever again. They don't want anything from me. Every time I try to do something nice. I get slapped in the face. Yes, my mother is grieving for a 60 year old marriage. But, she doesn't have to take it out on me. My husband says to keep calling just turn the other cheek. She doesn't act that way with my husband. I want to discuss her bad behavior, but I know she will not co-operate. Maybe I will just let my husband do the calling and check on her. It is too painful to be slapped in the face each week. My concern is not so much the bad behavior, but the type of care my sister will end up giving my mother when my mother needs it most. Or will my mother be the caregiver to a psychotic mentally ill person whom she can't handle? Will I be contacted in a timely manner when things go south? Do I just wait until matters get worse? I have expressed my concerns to my mother, but she believes strongly that she is in control, and my sister is cured. She is sure that my sister can be the ultimate care giver.