AIBU to be upset that DP slept with my sister-in-law?

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Name changed for this one - it's come as a bit of a shock!

I met DP through my brother as they are very close friends. I've always had a bit of a turbulent relationship with my SIL as she's a bit of a drama queen and loves a good argument and that is the absolute opposite to how I think life should be.

The 4 of us have become quite close over the past couple of years, with DP being such good friends with my brother and me making a real effort to be friends with SIL. We have had a few nights out recently and she has made some comments that bothered me, and has also been extremely flirty with DP. To cut a long story short, it led me to ask DP if anything had ever happened between them both and it turns out that the first night they ever met (over 15 years ago!) he slept with her.

Neither of them did anything wrong. They were single and met on a night out. It just turned out that she ended up seeing my brother and marrying him instead!

I feel absolutely gutted about it all though. I feel stupid that I knew nothing about it and really sad that DP didn't tell me until I specifically asked.

DP is absolutely not the flirty type and has never ever done anything to make me question how much he loves me. He said he just never felt that there was a right time to bring this up and it was so long ago and clearly meant absolutely nothing. It's come up because she was clearly angling to tell me.

Have posted in AIBU as I genuinely don't know if I am BU by letting this bother me. I feel almost betrayed in a way. Like they had a secret I didn't know. I can't get the thought of it out of my head. She has no idea that I know yet...

Oh Christ. No advice, but just wanted to say I would struggle with this massively. Even though it was years ago, single consenting adults etc etc. I think it would be the feeling of having been kept in ignorance.

Maybe she wants you to know as she feels bad she thinks you don't and wanted to push it so your husband would tell you.

I know if I was friendly with a woman whose husband I slept with, but that woman was my sister in law I think I'd find it difficult to tell her and cause her problems in her marriage but hate the fact i couldn't tell her, I'd want her husband to tell you. Which is how it should be,

Play nice, it may have been she felt shit about it and was trying to force you to ask him, if the situation arises make a comment and just say "listen I know about you and husband, don't stress it, it's no biggie" then end that conversation.

I'd struggle with this too. I wouldn't be able to get it out of my head, it would make me feel sick. But I'm a very petty person!

When we moved house and changed schools I got on really well with one of the Mums in DD's class. Then DH did the pick up with me one day and recognised her as his sixth form girlfriend. He had mentioned this girlfriend as someone he had had deep feelings for, but messed things up with, so I felt a bit strange about it.

They are pleasant and friendly to each other when they see each other. The woman and I never deepened our friendship after that though, which was a shame in some ways, and our children are not friends (different sexes).

I'd be tempted to wait until she's being flirty with DP, catch his eye and mouth "Are you okay?" in a way that she can't miss. If she counters with one of the 'DP and I have history' type lines just counter with Lonnie's "You do know that DP and I have no secrets?"

Are you your DP's first really serious relationship? Because I can't think why your relationship is so threatening to her. Is she jealous of your relationship with your brother as well?

I think you need to let it go and leave it where it is. In our wider group of friends is my first boyfriend who broke my heart. My DH knows who he is and talks to him. Everyone knows we have previously slept together, I presume the ex bf wife knows but I couldn't say 100%. It's just not talked about. My DC do not know though!. It's a bit weird and cringy but we get over it. That said I don't flirt with him. But we do get along

rollonthesummer she doesn't ever say these things in earshot of DP or my brother, it's always quiet and said in jest, but just for me to hear. We have recently started going out more (just the two of us) whilst the men have been away or on golf weekends etc, so I think that's why the comments have started lately as she's had me on my own a bit more.

Liska - DP and I were both married before and both in long term relationships for over 10 years. We only got together about 5 and a half years ago. I think it was obviously tough for my brother - his sister and his best friend getting together! But he dealt with it great as he knew we were so compatible, and she just never liked it. It was as though DP was 'their' friend and I had stolen him or something!!
Which I can understand to an extent. Hence me making such an effort with her so that the 4 of us could be close, even though she was trying to cause trouble in any way possible.

FoxInABox, FannyDeFuzz, ShoutOutToMyEx - thank you. It just helps so much to know that I'm not being completely crazy for feeling this way. Even though he's technically done nothing wrong, when I found out it felt like that feeling you get when you realise someone has cheated or lied to you and you just feel sick to the stomach. I do think in time it will get better, but after reading all of the positive comments and support I feel stronger (and more justified) in being able to say that this has really hurt me and I just need some time to process it.

I'm going to do what you all suggest though and next time there is any sort of comment from her I'm just going to make it clear that we don't have any secrets and I think it's quite pathetic that she needs to keep going back to something that happened so long ago.

Thanks to everyone for taking the time to post, it's helped so much.

I feel a bit conflicted about this as we live in a small community. One of my DC (a DD) has just come back to live here again with her own family and DH, not from here. A lot of her old teenage crowd are still here or come back to the area and they've all remained friends, some since infant school. Her OH particularly likes a friend who she had an on off thing with when they were teenagers and students though they never went out. Her DH knows and has met her first love (who doesn't live here) but is she expected to tell him everyone she had some sort of liaison with over 10 years ago? Her DH has a past too and is not the jealous type and hasn't asked so I'm not sure it's really odd that the situation in OP's post wasn't highlighted...