"We almost didn't see them Christmas and we didn't see them at all for my grandsons first birthday. My grandson was ill but even if he wasn't it was made clear they wanted to spend the day just the three of them."
Well then, take heart! This is very common today, to my understanding. Not what you and DH are used to (me either) and maybe not what most people do, but increasingly common. Especially if both parents work or one parent works long hours and they feel the need for that time together as a family unit. IOWs, it's not something that's only happening to you and yours, if that's any comfort.
"They spend most holidays and birthdays together, sometimes with her family and rarely with us."
Ok, I get that it can hurt to see that they spend more time with her family than with you people. But are you sure that's the case? The difference between "sometimes" and "rarely" is so often in the eye of the beholder. Her mom might see it the other way around. And YDS and YDIL might think they're dividing things up about evenly.
Then again, if their relationship is a little better with her parents than with you people, at the moment - and from their different experiences with the living arrangements, I imagine it is, I'm sorry to say - then that may be why they tend to spend more time with them. As your own relationship with them gets more comfortable, that MIGHT change.
But if it doesn't, please try not to waste time comparing - especially if it's a matter of "sometimes" versus "rarely" - it will do nothing except cause you hurt.
"'I'm hurt that we have never been trusted to be alone or look after him."
Again, chances are that this ^ will change, too, as time passes and GS gets more comfortable with you. Maybe not but it's possible. I know it may be hard, but please be patient?
"They won't hear from me how much it hurts though."
Much better to vent about it here.
Have you thought about apologizing as some of us suggested? For taking over wedding preps, disrespecting YDIL's wishes about laundry, etc. and refusing to listen to YDS? It won't change things overnight, but it may help you begin the repair the damage done to your relationship with them.