It's really up to your future sister in law and her family to establish the limits on the invite list if her parents are contributing or for her and your brother to have that conversation with your parents.

I take the insurance comment to be more than just a point about you personally and it sounds more like a sentiment about "the change" that resounds with many. My dad held off on his divorce to drag it out past my half sister turning 26 because that too he viewed as a wiping of hands in money. I personally found it such an oddball age of demanded mandated coverage for a child. On one hand I get it, on the other hand....??