When grandparents help out there is always problems.
Is it fair, that she has your sisters kids and not yours...not really.
Would it be fair if your sister had io reduce her hours since you have decided you don't want to be sahm anymore? Not really.
Is fair that your mum is looking after children 4 days a week when she has already raised hers? Not really.
My mum had my dd one day a week at her request, while I worked. The rest of the time she was in nursery or with me or dh. She didn't have my son at all. When dd was 9, my dbro had his first.
So mum was nearly a decade older and had suffered ill health. She was no longer having dd.
Dbro kept insisting it was only fair that she had his children (they now have 2 under 2) one day a week too. Except his wife is a sahm and her mum has them 2 days.
Even mum does have them they are always asking for favours. Like can mum go sit with the kids so sil can go to the supermarket.
Mum has had to put a stop to it. It's too much but all dbro can say is 'that's not fair' but our situations were different. Mums healthy was different. Fair and equal aren't always the same.
It's seems to me that once a parent offers to help with childcare they put themselves in the middle of a tug of war between their kids.
What if your mum decides to make it completely fair and look after no ones children?