We've now had 3 parties for DS since he was born. At each one my in-laws have sat at the side of the room while everyone else was milling around/ getting stuck in. And there they sat for the entire parties, only talking if people made the effort to go over to them. It sounds odd but it came across like a snooty royal couple not wanting to stoop to everyone else's level.
The first two parties were held in a local hall where the only seats were built-in, so I assumed that they were tired (they're in their 70s) and preferred to rest their legs, and I actually felt sorry for them that we couldn't move their seats closer to the action.
But we held the third party in our home, so they were right next to everybody. DH and I made sure we introduced everybody to them as soon as new people arrived and engaged them in conversation with said people, then...nothing. Not a peep, apart from MIL instructing my friend to use a napkin for her cake (wtf??). They remained seated the whole time, and then left very early.
I can understand it more from my FIL as he's often rather uncomfortable in social situations, but MIL is really sociable and incredibly chatty with her own friends and strangers. She loves organising social events and they're both physically fit and very active.
When we go to parties at their place we always make an effort with everybody.
DH and I both thought they have behaved rather oddly and rudely, and tbh I'd rather they didn't come to future parties - but you can hardly omit to invite people to their grandchildrens' birthday parties, can you?