Oh Ellie Anne I feel for you sweety. but you need to understand that it is not that DIL parents are favoured it is just that Mums parents are usually the ones first called upon for some reason.
If I were you I would have a little chat with your daughter in law and actually ask, now that your parents are moving closer, will you still need me to take care of the children two days a week. Communication is the key in this situation and I feel that you are valued more than you think you are and a chat with your DIL or even your son will clarify things for you.
How lucky are they that you baby sit two days a week, my DIL asked this of me and I had to say no because I still work so I know my DIL would swap you for me any day 
It is sad that you can not speak to your husband about this, but it is something that you can resolve yourself.