I know this is in the third-ager forum, and I don't really qualify as a third-ager, but as a momma to two young ones that is asking her MIL to baby-sit, I thought I would offer a different perspective.
I do ask my mother-in-law to baby-sit, maybe 2-3 times per month. She can't always do it, and it's not over night very often. It's nice for me to not have to pay a babysitter and I know the kids love her. She tells me no when she can't and there are no hard feelings.
With our family situation being the way it is though, I can't always invite her to do more with us and the kids except for when I need a sitter. Hubby works second shift so we don't see a lot of him. So on his three days off, we want to spend time together as the four of us, because we only get that approximately 6 days out of the month.
I would love to have invited grandparents to Wesley's Christmas program at preschool, but that would have been 6 grandparents and 4 great-grandparents that would have had to have been invited and our preschool asked us to limit it to parents/guardians and siblings due to a lack of space.
And there for awhile, when I was really stressed and busy with the new job, pregnancy, Wesley, etc, I did shut my mother-in-law out. It wasn't something I did intentionally or that I had even realized I did. It wasn't until she called me out on it (in a very nice and gracious way) that I realized she was absolutely right. And since then, I've made an effort to call her more frequently just to talk, to have Wesley talk to her, etc. Since I can't have her to the preschool programs, I asked her to take my place as a 'preschool pal' on Wesley's day as snack leader so that she got to go and spend the morning at preschool with him.
So to sum it up, I would advise to talk to your daughter-in-law about how you are feeling. Do it gently, with no accusations. I don't know about your DIL, but I know that I feel like I am being pulled in about 90 million directions and if MIL had came at me and just bluntly said everything she had to have been thinking, I would have gotten defensive.
Good luck to you!