bread and honey - please don't stereotype in-laws. I am referring to your comment "they are in-laws - goes with the territory" I find this rather offensive as a mother, gr/mthr and one of those humans unfit to stalk the earth a MIL!!
We mils/gr prts can't do right for doing wrong. Either it's the pils aren't doing enough, never bother to help out, or they are doing too much, as in this case, and wanting to care for the children. I may be wrong but the Op seems to be saying that the grprts have dd1 a weekend every other month (so I weekend in 8 - I don't call that excessive) I think the most important thing that no one has mentioned is does the child want to go and stay, because if she does, what's the problem. Children have the right to have a relationship with their extended family (especially grandparents) - If she doesn't want to go, then that's an entirely different matter.
Some of you dils are so busy piling in with negative comments about in-laws you don't stop to think of what the child wants.
I certainly think that the 12 week baby is far too young to be spending a weekend away or even a day. And if they live half an hour away, they could of course have the older child for the day, but again does the child want to stay overnight. I can't think she is going to miss out on this precious family time by have 1 weekend in 8 with her g'prts.
I never ever make comments to my dils about how they are bringing up their children and so I don't agree with what they said about the sleeping.
Anyway I hope some of you young mothers remember that in all probability you will be a MIL one day and if you have a dil, let's hope she is more tolerant and kinder in nature than many of you seem to be. OK I am now going to don my hard hat and retreat ...............