re: I just started vacation, and my wife just allowed her sister

Speaking for myself, the closest I've been to divorce was when in laws came and stayed for more than three weeks.

If it happened again a preemptive divorce would probably be less painful.

Having said that, did you mean it when you offered to let her sister stay "for a while"? You might have been trying to be nice but jesus, that is the kind of open ended invitation that can only end poorly.