My parents have DD two days a week and have done since she was 8 months, too. Its an ideal arrangement - she loves going there and we have never had any tears from her or worries about it whatsoever. Plus I think my parents love my DD in a way that childminder/nursery worker never could, no matter how caring.
So in principle I think its a great idea to have granparents involved one or two days a week. However my parents only live three miles away so it means that:
(1) my mother/parents don't need to stay with us
(2) they can carry on with what they would normally be doing (eg shopping, visiting friends - DD just goes along too).
So DD isn't much of too much of an upheaval in their lives - although two fulls days is of course quite a commitment and I am sure they need a break at the end of their two days! But she does come back to us at the end of the first day, so they get some down time.
There are practical things - ideally you all have to take holidays at the same time otherwise your DD and her DH will use up all their leave looking after GDD while you are away.
I just think you having to be away from home two days a week (leaving your home comforts, your partner (if any) and everything you've got on your to-do list) and staying in their house regularly in order for this to work, may be a step too far.
If they were both totally behind the arrangement, then maybe it would be OK. TBH I think they ought to have come to a common view between them before discussing this with you. If your son in law isn't happy, then it isn't going to be great your staying there two days a week when it wasn't the arrangement he wanted. Your DD and son in law need to resolve this between themselves.
The other thing is, when they have baby #2, how would you feel about having both of them? Because if grandparents have the first baby two days a week and it works out really well, your DD will probably want the same for the next one! I know we do - and we are lucky because my mother is retiring and so says she is happy to have two (DC2 due in Oct).