D.W.
It sounds like your mom's health issues have made her bitter, and she's taking it out on you. It's not fair, but none of us can walk in her shoes either to understand how she's felt about the past 30 years, so I don't want to pass judgment other than she's not treating you well and empathizing with your personal responsibilities with your own family.
ALS - that's a tough disease, and I'm sure it's really hard to watch your sister go through that.
I do understand how you feel about feeling selfish. I had a hard time coping with having a "good" cancer diagnosis when other people I knew were diagnosed with terminal cancers, poor prognoses, etc. I struggled as a survivor and had to seek therapy when I began to hear of other people's relapses and passings.
Instincts will be your strongest asset right now. It's OK to cope the way that best helps you get through it. People will want to help. Don't be afraid to tell them what you need 0 sometimes people step-up with kind gestures that are poorly timed, not needed, and we have a hard time saying, "What I really need right now is........".
Make sure to take care of yourself during this time. It will hit you like a ton of bricks when both finally pass. I hope you have a good support network and husband to help you grieve and gain closure.
Best wishes.