I don't know what to say, really. My deepest sympathies, and you son-in-law sounds very immature and insecure if you pardon me for saying so. My mother-in-law and me are of very different political opinions about most things (she had quite a severe case of TDS in 2016/2017 despite us being swedes, while I just quipped 'Well, in a democracy anyone is supposed to be able to be eleceted by the will of the people') but that has never stopped us from civil discourse or associating as family. I hope he grows up and comes around to apologising which he should hat in hand, because regardless of opinion that kind of behaviour of his is one you're supposed to have outgrown before puberty, really.