"Standing in front of the children and telling several of them they are rejected by someone or not permitted to attend an event will not help foster healthy sibling relationships"
No one is advocating for you telling the DSc that your parents reject them. Especially since yourself don't think that's the actual reason and that you don't know the actual reason but logical dictates it's not DSc. I think anyone with a brain can see that. I'm sure there are evil people out there who hate DSc but that evil tends to be overall evil. And from what you have posted that's not who they are.
No one is advocating for you to exclude dsc from parties events ect.
But what you seem to be missing is that your son is now by proxy not able to see them as frequently because your feelings are hurt because of the fact he can't see his biological grandparents without DSc in tow. So your being awkward around how many times they can see him or that you can't get to theirs because of x when really you want them to come to you and they won't as your DSc are there.
DSc won't miss out on seeing their grandparents or the frequency of contact change because of your son's existence, but your son and baby will ? Who looses more here ?
I would love to say it's cut and dry this situation or that your parents are indeed evil and therefore awful people who don't deserve a relationship with their grandparents. That's a easy but lazy answer that ignores the facts stated.
Your a mum. Your the only person who can and will advocate for your son, your DSc have two parents who I assume do advocate for them and now you so 3 people.
Your sons literally got you. And since your ex was a human trash bin (I think you mention he was abusive previously so hen the name but willing to be corrected on that point if I misread that) that's the only other people he's got is your grandparents and you.
It seems to me that your prioritizing your we must all be a "happy family" and DSc potential upset feelings over this (which hopefully is potential and you haven't actually told them this) over your actual child's feelings over wanting to have a consistent relationship with his grandparent. You could facilitate this in a way that didn't effect DSc, and allow your son what he wants (just by letting them take him over to theirs for a bit of one on one time and same with baby) But if you don't well that's your call.
But I have seen it time and time again and you may find your DSc reject the notion of blended family when older or even now and that you will be forced to accept that under they didn't chose this. Well neither did your son or your grandparents.
Btw I'm firmly of the opinion that blended families can work but sometimes personalities clash and you can't force people to believe in the same things as you (DSC, family) and no one has to be evil for that to be true
Watch this board see how many adult DSc say you know "I don't see my dads second wife or family as anything to do with me and why should I" and how many people encourage that. Which tbh is fair, but when you make sacrifices in the name of family, remember the scales won't always balance how you like. And the only tangible hit that's happening right now is your son missing their grandparent.
I only say this because someone's gonna be along to say your grandparents are evil and cutting contact and I'm trying to be empathetic to you as you must feel like a human sandwich and are probably exhausted. But by saying well I can't or won't drive 15mins away they can come to us but DSc you are by lack of options stopping them see their grandchild.
My ex didn't drive, I dropped my dd over and collected her at the most inconvenient times regularly so he could see her. I could have said I don't have a car (sometimes I didn't) I could have said walking isn't safe or multitude worth of excuses that would be valid and his contact would have dramatically dropped. He hurt me, he married the ow he had been cheating on me with after the loss of our son. I could have punished him because he made some fairly awful moral choices and let their relationship suffer because he bought it on himself. But my child would have suffered more.
Anyway look I'm out. I'm not into stressing out a pregnant women and frankly this will be totally against the bell curve of comments you will get many I assume from people who don't have blended families. I grew up in one and these things do come back to bite people in the arse. My sisters (half) mum wasn't a great women, but even I have some sympathy for a women who is now nc with her adult daughter because she didn't put her child first and doesn't get to cuddle her grand babies. Before anyone asks (I have tried talking to my sister to little avail) she says her mum had her choice. You still have yours...