This is a bit of an issue in our family.
I have always tried to share things roughly equally and ensure both sets of grandparents get as much time with kids as each other and don't feel left out. Alawys asking one to go on holiday if we've asked the other etc and always asking them both over.
And I've done this despite finding my In-Laws deeply irrating, because they are my Dh's parents who he loves (but also finds irratating) and because they love the kids and the kids love them (currently oblivious to their irratatingness!)
I am close to my parents and enjoy their company but think this should not be the only factor.
Now, my SIL has had baby one year ago and has been on holiday with her parenst for a week and then stayed with them for a week and spent one night with my parents on the way home. My parents feel rather shut out and I feel quite hurt for them.
Also as the mother of boys, I am quite concerned that this fate lies for me, particuarly afer rading many posts with views such as 'it's your baby do what you like who cares about thier feelings' (paraphrased).
Surely we should also care about the feelings of others in our family even we if don't find them easy?
I know there are many whose In-Laws are so intolerable that they cannot and should not be endured by anyone, but I'm thinking more of the 'I just don't like them much' kind.