Yes, totally agree with you. You are NTA. I also have a brother who has a child, and they ask me and my partner to watch him, but it's really like once a year and they are very apologetic about it. "Sorry it was so last minute couldn't find anyone else, thanks a lot!!!" And they prepare all the foods and activities or give us money to take my nephew out. And I do help out. But nobody is supposed to help out with kids every weekend, if the kids are not theirs! Nobody is really supposed to help out with others' kids, period. I would maybe get your brothers' point of view, if he rarely wanted help and it was very last minute and he prepared everything for them to be comfortable, but even then, Rene is not obligated to agree to take care of them.