Thanks, Gyps...

I hope that my long-winded version says pretty much the same thing - What works for you works for me.

The thing is, now that I've put it out there, I will make a point to not talk about x or the sibs with the in-laws at all. (I had made the mistake of asking MIL if she knew her son had come to town over the weekend - she didn't). If they bring it up, I will politely change the subject; just want to remove myself from the equation entirely, so that they can all choose to do what they'll do. I think that they have all begun to agree with x that I "turned his parents against him," and they are all pretty pissed about it. At this point, I don't really much care if they have made that judgement about me - it's a shame, but I can live without them.
It is just sad to see for my in-laws. I don't understand how my xBIL can make that decision to literally drive past his parents' house with their grandson, who they don't get to see very often at all because they live over 2 hours away.
It is so obvious now how dysfunctional their family dynamic is. All of this is done, judgements made, without anyone talking to relevant people about what is truly going on or bothering them. It is not surprising, then, that x went through his unhappiness without ever saying anything to me until he had already decided it was over.
No, instead, they talk to each other, about each other, behind backs and in whispers. And Grandma just wants everyone to get together and pretend that nothing has gone on so we can all "have a nice visit."

I have to say, I don't all that mind removing myself from that drama.