For them to say they don't look after kids "anymore" implies to me that they feel they have been taken forgranted, and perhaps do feel used as free babysitters, as though this is their duty as Grandparents.
Maybe they resent having looked afer your neice and nephew "all the time", and made a decision not to be doormats anymore. Personally, I would rather pay someone to watch my children than have someone in the family mind them under duress, which may affect the relationship in a negative way.
By all means ask whether the kids have made a nuisance out of themselves in the past, but I wouldnt expect them to reverse their decision, it does sound pretty final. You're right in thinking its not fair to have your parents watch them every time, which is why it would be better to get a babysitter you can trust.
They don't have to. Yes, you can find out why and respond with grace, but if you don't want to, you can just let it be and respect their decision not to stay with the children. I think we must honor people's choices no matter who they are. The fact that they may care for other children in the family is totally up to them. It is their life and their choices. Take what you get, and accept what you don't.
