Hi birdpumpkin, you tried and it mattered...it counted!! You were there "by her side" at a time she probably felt vulnerable, shaken, drained (?) with everything that had gone on for her. And that was an excellent time just to be there for her, a time she may well have needed someone close to her. So you and your sister could have made a difficult time just a little easier for her.
As for them calling on your sister and feeling "left out" though, well maybe they felt that she was close by, she often visited, and they didn't want to inconvenience you/or kind of "second nature" to phone her- nothing personal.
But you can still turn that around a bit, you wanted to be there more often anyway which I'm sure will make a big difference. And maybe you and your sister together can make a great "tag team" in helping your mom in her recovery. Today mattered, and so can tomorrow, and the next day...........
Alison