I’m thinking those shock collars would also be great.
Though there’d probably be some small account of kidnapping etc and then some other plan to force the collars on their necks etc.
However once these small wrinkles are taken care of then I can guarantee that a few zaps from those collars will sort it out quick sharp.

(Alternatively, and boringly, just don’t go to family get togethers until they learn to not be twats. I still think the shock collar is better.)