You know...what she is doing is VERY rude behavior. She's not even giving you the common courtesy she'd give an acquaintance or a stranger on the street. THAT is what would piss me off, and does piss me off for you.
To me this behavior is her trying to push you to do something, trying to push you to be the bad guy and end things. If she were truly wanting to get away and do some "thinking"...she'd go somewhere for the weekend by herself, or at least for a night by herself....not to her SIL's not to the OM...but all by herself. I know you already know this...but it's really hard to do clear & objective thinking when a biased opinion is right next to you.
