Taking Care of Mom, But What About My Sister?

Currently I'm caregiver for my 94-year-old mother. That's frustrating and depressing in itself but for now I'd like some advice as to what to do for my youngest sister. She will be 50 years old this summer and has some disabilities. My parents never really sought help for her except taking her to the clinic when she was a child. It wasn't until years later they tried taking her to SS to get some kind of disability but never followed through. She had epilepsy from the age of 2 until her early 20s. She was put on several medications and I'm not sure if those contributed to her disability or she was born that way. My mom was sure that right after giving birth, she heard a nurse exclaim that she dropped the baby but my father didn't want to pursue it further. Anyway, my mother is very concerned now as to what will happen to my sister after she's gone. I don't know if I should contact a social worker, or call the office of Mental Health as someone suggested. My sister who's on Medicaid did go to a neurologist who said she was fine which I find hard to accept, especially as he doesn't know her habits. When my mom passes away it seems to be expected that I would take care of my sister but I was hoping to finally have a life of my own and finally some peace of mind. My sister's never worked but has gone on numerous job interviews. I'm sure prospective employers note her speech impediment and other characteristics but of course won't mention that to her. Currently she's doing volunteer work at a local hospital for 2-3 hours, three times a week. Anyway, any advice would be greatly appreciated. I may call the Center for Aging as well but wanted to see if I could find any advice on here. Thank you.