Remember…they Are your kids and totally belong to you… It is not your mother’s responsibility to watch them… If there’s anything she doesn’t like about it or you don’t like about it it’s time to find other help
I would just find a new arrangement for someone to watch the kids. Deffinetly before it’s to late and she hits her limit and you don’t have time to find a replacement. Don’t let her get to you though. There’s always gonna be someone with an issue. My dad was a single dad who worked night. My sisster was like 16/17 and I was 12/13. So she was old enough to watch things and we just slept anyway. He would come home from work take us to school and be up by the time we got home. Hardest working man I know. There were many comments. But all his kids worship him and hold no resentment.
Get a new baby sitter.
At the end of the day it seems like your mom is trying to nicely tell you that the schedule isnt working for her anymore amd that she is concerned. She is not the parent of the child and one of the two of you needs to make a sacrifice at this point and be willing to dedicate extra time to your children. 
Sounds like you need a new sitter
Working nights is not for people who have small kids flat out. I work nights with a teenager and it is beyond awful. I would never even consider it with young ones! Your mother already raised kids she has every right to be upset!