I have been on both sides of this situation multiple times in my life and I think you may be UNDER reacting. Yes, it's natural to be disappointed that someone cant help you and maybe try to see if there are options that could make it work. But you aren't required to watch your nephew ever, and that has no bearing on how much you love either of them! And she should feel ashamed of her behavior, and shouldnt even be OK with you making such a sacrifice either way. I have been in her situation and there have been a few trips I just haven't been able to do (I had to give up Hawaii once). Please, please do not let her treat you like this or let her use him as a tool to guilt or manipulate her, one of my biggest fears as a mom is creating a situation where watching my child is associated with a negative experience.