Heed HellFire’s words:
That's a common lie cheaters typically tell their BS, when they're caught.
Regardless, no,it doesn't matter. She had sex with another man,and is having an affair with him. I bet that's all that will matter to their spouses.
Also..affairs are not one big decision. There are literally 1000s of decisions, and actions,leading up to,and during,an affair.
Unless it was a ONS..it's almost always "more than once."
Your sister has "crossed the Rubicon". Period.
When I confronted my WW, I had the name of a suspected AP. When I mentioned the name, she said she "didn’t know who that is". I reminded her he was a coworker of hers at the time of the affair. The next day, she admitted she had slept with him but it was "only once and I was really drunk". Now it is true that she only slept with him once (I have reason to believe this I won’t go into here) and she was drunk. BUT, they had been flirting for weeks at work in the smoking pit (she was supposed to have quit smoking too. Liars lie.) and arranged to meet at his house while his wife was out of town. She was out drinking with another friend when she "got the call", drove to his house, screwed him and sat talking on the porch afterward. Hardly a "drunken ONS". Like HF says "literally 1000s of decisions". This interlude wound up being discovered by AP’s wife, AP and my wife got in trouble at work, AP got divorced (serial cheater) but no one bothered to tell ME at the time (I’m looking at you OP)! My WW had what she calls multiple ONS. None were drunken hookups in a bar with a stranger. ALL involved days or weeks of flirting, inappropriate behavior culminating in one instance of intercourse whereupon my WW would promptly "ghost" AP. WW was like a black widow pursuing men to see "if she was attractive enough to sleep with her" then when "validated" she moved on. Sick game.
More HellFire wisdom:
There is no going back to normal, when a spouse has cheated. Even if it wasn't exposed.
A BS typically knows something is wrong,but they don't know what. They're told it's nothing, which the BS knows isn't true. Or, they're told it's their fault. So the BS wears themselves out, trying to woo their Ws.
Every person Deserves to make their life choices, based on reality. They deserve their agency. When they are kept in the dark, they make huge life decisions with a person who can't be trusted to do what's best for them.
Right now, you are an accomplice in their affair. You know your BIL is depressed, and full of anxiety, because he's being lied to,and he knows it..he's just not sure what's going on. It's a scary place to be.
Another thing to consider..if she gets away with it,she will do it again. Why wouldn't she? She has zero remorse.
My WW was a serial cheater. She had a "tell" where she would treat me cold, withhold sex and tell me "she didn’t think she loved me". This was ALWAYS after she had cheated or started inappropriate contact with an AP. Horrible people cheat. She isn’t a horrible person. There must be a reason. Sabotage the relationship to "prove" she is unhappy. After D-Day, when I learned about her affairs, every time I suspected her of cheating, she was. Always lied and denied. But then she "honed her craft". I never had a clue about her last 3 affairs.
These words are the MOST impactful for me:
"Every person Deserves to make their life choices, based on reality. They deserve their agency. When they are kept in the dark, they make huge life decisions with a person who can't be trusted to do what's best for them."
I caught my wife in an EA in 1986. She left but then asked to return. I agreed if she went NC with her EAP. She agreed. She didn’t. My daughter was conceived while my wife was in an active affair. I had to DNA test my 35 year old daughter. That was pure hell. Now, she IS my biological daughter. But she was not "conceived in love". That’s a HUGE life decision I made with incomplete/inaccurate information about my life. I would have never stayed with my wife had I known she was continuing her affair let alone had another child with her. My WW had 7 more affairs after that one. Cheaters cheat. My life was stolen from me. Other people knew, no one told me. Don’t be "that person".
Your sister has a cavalier attitude about cheating. It’s highly unlikely there was only one instance of sex (most men will bail if that’s the case) and it’s also highly likely this isn’t her first affair and with this attitude, will definitely not be the last.
I get you are risking destroying your relationship with your sister. She is showing you her "true self". What kind of relationship do you want with "that person"? You can save your BIL from "living my life". You can save your parent’s livelihood and reputation. Painful as it may be, I think you know in your heart what your decision SHOULD be….
My handle causes some people angst. It is very much "tongue in cheek" for me. But if it helps, call me AStolenLife.