Hello,
I don't know if you will find this helpful, but wanted to write and give you my perspective. A year ago, I was diagnosed with Stage 4 Cancer at age 42. Perhaps your sister is experiencing some of the same things I was. She is faced at an extremely young age with her own untimely death, no control over her own schedule, emotions (at times), and she is now the topic of family conversations(whether she is present or not), and what is in her best interests. Add into this situation, a possibly over-involved, or overly emotional (understandably, having already lost another daughter) parent, and the potential brain functioning issues, and it is overwhelming, to say the least. I would recommend that your sister see a counselor or psychologist at her treatment center, that is just for her, therefore, she may be able to unload some of her anger toward your Mom. Sometimes our families expect us to be "who we always were." This is not always realistic. When you are facing this kind of turmoil, sometimes the worst comes out. This is not always the worst thing, as there may be a need to get some of it out.
In turn, your mother should also seek counseling, so she can maintain a healthy relationship with your sister, for as long as she is here. She should definitely not allow herself to be your sister's 'whipping boy,' giving herself time away to recharge her batteries. Another way to minimize anger and confrontation is for her to ask your sister what she wants, or her opinion, rather than treating her like a child. The loss of independence is one of the most frustrating things about a terminal illness. Hope this helps, I will be in prayer for your family. God is the God of miracles~