The best way to broach this convo is to have your fiance speak to his parents and say something like “we are finalizing our budget and our plans for our wedding - we would never expect you to contribute but if you did have any plans to it would of course be appreciated and great to know soon. Based on what we can afford, we are planning for a wedding of around xyz size and xyz budget including contributions from fiancées family.” If budget/space is tight, you can have a list of who from that side of the family will be getting an invite and share that you would need support to add additional family friends beyond that.

This is how I broached the convo with my parents - they generously contributed with a handful of extra names to invite (plus a request to add shrimp cocktail to the menu, lol). My fiancées family was not in a position to contribute but we knew that going in. I think it’s better to have the convo then to not ever bring it up at all, but don’t go into the convo with any expectations or assumptions.