PD wife and sister in law at it again

[Sadhubby]

My wife and her sister have this extreme relationship where they are either best friends or not on talking terms. Usually BFF goes to not talking after a few days: there will be an explosive argument, then no contact for months. Then they start talking again as if nothing happened. Her sister is incredibly sensitive, creates drama everywhere she goes, insists on false realities, throws tantrums, and never ever apologizes.

It has affected every single family holiday we have ever had. We are now staying with my mother in law during lockdown my wife's maternity leave.

I usually don't care and enjoy the quiet, but our son really gets on with his cousins and now they are not allowed to come visit even though they live across the way from where we are staying. My wife won't leave her corner, either. Now everyone has been walking on glass around them for weeks to accommodate the stupid feud.

I'm not really looking for advice on this situation, it seems pretty pointless to tread in that minefield. I'm just looking for a place to vent, really