'WIBTA if I told my future SIL the truth about my brother/her fairytale fiancé?'

"WIBTA if I told my future SIL the truth about her fairytale fiancé?"

My brother Paul (31M) is getting married to his fiancée Mary (30F) next October. They met at work about two years ago—both lawyers—and claimed to be “inseparable” from the start. Real fairytale vibes. The issue? Paul was in a 10-year relationship with Bella (30F) when he met Mary.

Paul and Bella were deep into their relationship—renovating her home, planning to move in. Our families were close, we all thought they were endgame. They supported each other through college, traveled, had baby names picked out, and seemed genuinely happy.

Paul’s a big skier, and usually went on group trips. But that year, he went “solo” and started acting distant, especially when Bella came up. We thought he was planning a surprise proposal—he’d mentioned it a few weeks earlier. Then one day I saw a coffee cup in his car with “Mary” written on it. He brushed it off, said it was a mix-up at the café. I didn’t think much of it then.

Right after New Year’s, Paul abruptly broke up with Bella. No explanation, no closure. He blocked her on everything and left stuff at the house they were renovating. She was heartbroken, reaching out to me and our parents, sobbing and desperate for answers. Paul wouldn’t talk to her—or us—about it.

Less than a week later, Paul told us he was seeing someone new. Tension grew. Eventually he said her name: Mary. My stomach dropped. Same name from the coffee cup. That’s when it clicked—he’d been cheating. For months.

Then we met Mary. Paul and Mary would giggle about how coworkers used to joke that “Bella wouldn’t like” their flirting. Like it was funny. Mary came off arrogant and loved stirring up drama. The more we saw her, the less we liked her. Paul is disgusting for what he did but she’s not innocent—she knew Paul was in a relationship when they met.

Here’s the twist: I don’t think Mary knows Paul was still with Bella so far into them dating. From what she’s said, it seems like Paul told her he and Bella had already broken up months before they did. I still feel awful for Bella. And Mary? I kind of want to give her a reality check. So… WIBTA if I told Mary the truth?

What do you think? WIBTA? This is what commenters had to say:

Suspicious_Fan_4105 said:

Would you telling Mary have any effect on their relationship? If Paul and Mary would giggle about what the coworkers would say how Bella would feel about them flirting, Mary absolutely knows he was with Bella, it’s just that neither one of them gives two shits about Bella and her feelings

joxx67 said:

She knew and didn’t care. Karma will get her because once a cheater, always a cheater.

Bergenia1 said:

So Mary is an affair partner? She knows what she did. She knows it was sleazy. Your brother will cheat on her too. It's obvious. I don't think there's any need to tell a side piece that the man who's cheating with her is in fact a cheater.

SnooWords4839 said:

Tell Paul in front of Mary, Bella wants to know if he wants anything he left behind, when he left in January.

Queenofthekuniverse said:

Well, you could be really sneaky about it and say something like, I’m really impressed that you are so confident in your relationship with my brother, considering that he was still with Bella for about a year when you started dating. (Or however long it was) I mean, you don’t seem intimidated by that old adage that if they’ll cheat with, they’ll cheat on you. Good for you, Mary!

It would probably be smarter to say nothing, but I’m a petty old lady…

crasho7 said:

No. Tell her. You have nothing to lose. Do it in front of your brother. Talk about a "thing" that happened right around the time he dumped Bella in "x" month...

Strong-Conclusion-52 said:

Does poor Bella know?

And OP responded:

Yeah, it was pretty obvious he cheated considering he went public with Mary not even a few weeks after he ghosted Bella. They have so many mutual friends too. It was horrible. Bella thought we (me and my parents) all knew Paul was cheating and didn’t tell her during it.

In the comments, she clarified some things:

Just wanted to clarify a few things based on some of the comments I've been seeing:

1.) Paul is my older brother. I think what he did to Bella was absolutely vile. That said, I can’t just cut him off completely—he's family, and he’s at every family function. My parents feel the same way I do, but there’s only so much any of us can do. Paul is a grown man who made his own choices.

2.) Mary definitely knew Paul was cheating on Bella with her. Based on the way she "jokes" about it, she clearly thinks she won some fabulous prize—like stealing a man is something to be proud of. That said, I don’t think she knows just how long he was in a relationship with both of them.

3.) I couldn’t care less about Mary’s feelings. Honestly. I would tell her flat out to come down off her pedestal with all those comments she makes about their “fairytale love.” She deserves to know the full truth.

4.) The house is only in Bella’s name. She inherited it. They were renovating it together to live in, but it’s hers alone.

5.) Paul and I used to be close, but I can’t even begin to understand what he did. I get that relationships can end—things happen. But how do you go from planning a proposal and fixing up a house together, to ghosting the woman you’ve been with for ten years? What makes it even worse is that to this day, he shows zero remorse. And that’s honestly terrifying.