I lost my husband to ALS years ago, and understand how you are feeling. After my husband's diagnosis, it was almost like there was a sign on our door telling people to stay away. I felt very abandoned, and got angry at first, but later realized that it was never because they didn't want to see him. It was because they couldn't watch or accept what was happening...so they ran.

I hope your sister can bring herself to seeing your mom, but this is a battle/decision she has to work out for herself. I wish you the strength you need during this time.