It sounds like the decision to be a vegan was his wife's idea (egged on by her sister), rather than his own, and he does not like it.
I would just tell my wife's side "Be as vegan as you want, I'm making myself a steak".
My daughter's best friend is a vegan, but she does not demand that anybody else be vegan. When she eats over at our place, we just make sure to make some veggie sides that she can eat. Sometimes macs & cheese.
If someone is eating mac and cheese, they are NOT vegans. They are lacto-vegetarians. My brother’s wife and their girls are lacto-vegetarian. Not as difficult to accommodate dietarily as vegans. Still, today was one of the few times since my brother got married 15 years ago that he got to eat a fresh, hot turkey. Usually we save him some leftovers. I would seriously advise anyone contemplating marrying a vegetarian to consider the consequences. It is just as serious as marrying someone from another religion...which my brother also did...her family is Hindu. Happy Thanksgiving everyone!
My best friend has been vegan for as long I can remember and she never pressures us to stop eating meat. Matter of fact, she is honored that we include vegan dishes as family dinners and even will fire up the grill for some meat for us meat eaters.
I never understood those vigilante vegans.