Babysitting for free??

I babysat my sister’s twin 1.5yo daughters for 6 hours on my one day off. It was nice to spend time with my nieces but they are a handful and I didn’t expect it to be that long (she told me 3hrs max) I also didn’t expect to be paid because it’s family, but I would have appreciated it if she offered. She and her fiancé asked if I could watch them again next week, if you were in my position would you ask to be paid or no?