Hi Idealist, Kudos to you for all you are doing for your sister. You have given her a wonderful gift, and that is the gift of understanding and support. You didn't mention if your sister has ever sought help from a therapist. If it's possible in her situation (insurance coverage?), she may benefit from seeing one.
You supporting your sister is terrific, and I hope you have some support as well and are taking good care of yourself.
Well wishes to you and your family.