Bee this struck a chord with me as I have been feeling the same lately so I know how much this hurts. My DS is 5 now and his grandparents have never had him on his own for 5 mins never mind babysat or had him overnight, that wouldn't be so bad if they were affectionate and loving with him etc but its taken 5 years to realise that they have no interest in him at all.
Its taken me this long to stop with making all the effort, inviting them over for tea, taking them to their house with handmade cards and gifts, anything to try and make sure he has a loving relationship with GPs (not helped by the fact that his other nanna, my mum, died when he was only 6months old, and would have been the best nanna and doted on him)
It has been hard, and it has caused arguments with me and DH who never says anything about it, he knows they will never change and i've really taken it personally and it breaks my heart for DS. Since a few incidents happened at xmas I have stopped making any effort, no texts, no invites over etc and it still makes me sad but i'm getting there slowly - i need to realise it is their loss.
My situation is different to you as there are 2 other GC with SIL and they are the same with them, so I know its nothing to do with us, they are just cold, distant horrible people who like to put on a front for their church friends fucking riles me no end!
I would say stop with the trying now as they are not likely to change, and it'll only make you more resentful and upset over time. You need to take comfort in the fact that you 3 are your family and as long as you are looking out for each other that's all you can do 