Things change over time. What she said a year ago might not apply now. Besides, you don't know why SIL needs this help. There are overt things and circumstances that the OP might not be aware of. It's easy to look at that from the side, especially from the OP's perspective. But you would get mad if you babysat one nibling, and then everyone demands your childcare for the sake of "equality". She doesn't have to divide her time equally between her grandkids. It would have been favoritism between grandkids if it hurt the kids. But it only takes more effort from the parents. Because not everyone is equal, as much as we want to believe. Some need more help financially, some need more help with childcare, emotional support, etc. I bet MIL assisted them in other things throughout their life, hence the wife's agreement and understanding. Op omit some things. It's easy to proclaim "I deserve that and that and that, because she gave it to someone else!". And dictate her schedule and what she does. But life isn't black and white.