Ummmm she's not your child. You need to be resting and looking after yourself! Do they expect you to look after her when you have your own new born child? Thats what there gonna think if you don't start saying no.
My temper is really short right now and i feel so bad because i love my neice to death. If my sister took care of her own child i wouldn't be in this situation at all!
Just tell them you have other plans that can not be rescheduled. It is not fair for you to be stuck watching a kid while that far along. I know I have a very short temper right now and don't even want to be around my own children much less someone else's. I have to deal with my children because they are mine but will not put up with others while o feel like c r a p;
Well really it's not your job to watch her you have your own baby to think about too