Will try to keep it short but....
My oldest sister lives 600 miles away from where my mum lives. I only live 5 miles from my mum. I have an arrangement with my mum whereas she take DS to school and also picks him up 4 days a week, so that I can work. On the friday mum works herself. Sis phoned beginning of week to say that she is going away this week (yest actually) and that her exDH is refusing to have their kids Thurs night even though he has then Wed and then from Friday to Monday. So sis phoned mum and asked her to come 600 miles to babysit. She has paid for her flight which is fine but her friend offered to have the kids for the night but she doesn't want to put her out but is quite happy to have mum lose a days work (friday) and leave me without childcare for the Thursday. Mum wasn't happy about it and initially said she couldn't do it then sis put guilt trip on her saying that me and her are in the same boat and solely rely on mum for sitter. I'm 5 miles down the road though not 500!
Sis's trip is work (or first day of it is work) and she's extended it for pleasure too which is why mum has to go down.
AIBU to be annoyed at her for expecting mum to drop all her commitements (and I'm not just talking about getting my DS) and travel 600 miles to babysit just because she doesn't want to put her friend out even though they offered?
Hope that makes sense