Without having read all the details, I can see both points of view. If it had just been a temporary emergency, it might have been okay (not obligated) for the sister to help out in a pinch. However, if she felt that her brother and sister in law were planning to rely on her continuously, that’s a different conversation. Especially if someone has consciously decided to be child-free, there can be an assumption by our society that you are automatically required to like spending time with children or that you don’t have any other responsibilities to tend to.