Hi Op
I get understand your wife's son Surley attitude is really annoying as heck,
Realistically you know yourself you are being unreasonable to expect to go along with your wishes to keep the peace by her going no contact with her flesh/blood Adult Son, under these circumstances,
I think going forwards,it better for your wife and her son, to see each other when you are either not around or and have set boundaries in place, how son conducts himself whilst in family home space,
I think your wife and her son could get into habit of meeting each other outside in a cafe/restaurant even just for cuppa and catch up, or in town in a shops,leisure centre ect,
even in Park, or beach if you live near coastal line at all,
Also if you or her struggle with setting boundaries in place dealing with her son difficult trying behaviour,
I think it may be beneficial to look on Internet youtubes mumsnet parent board ect for advice on how to put boundaries in place too.
What about your wife's sons father is he around?
Is he, any good influence in any way at all?
How does he find him (dealing with him too?
what his relationship like his bio father then?