Search Results: after the girl has cheated

Me: 16 (girl)

Brother: 26

Younger brother: 17

Girlfriend: 24

Younger brothers friends: 17 and 18

I found out by getting an anonymous letter on Facebook Mesenger in May about it. The sender blocked me right after I recieved it. I confronted them about it since theyre closer to my age and have mutual friends. The admitted it to me. My jaw dropped
I asked " how long as this been going on "
and they said " since like the beginning of this year ". She always joked with them in a flirty way in the past but I guess none of us took it seriously. They said it started at the new years party at our parents house. Our parents were gone to a New Years party. So it was my brother and his girlfriend me and oyr younger brother , us and some other friends. They said by midnight everyone was wasted. I remember it. My brothers were passed out I was still outside with some of who were still standing talking. She play flirted with them again but for some reason they flirted back and they started feeling on her and did some groping. But the next day is when he said she did the first deed with one of them. And it just kept going since.
She and my brother have been together since 2015.
He lives in the same neighborhood as us with his girlfriend.
Me and my other brother still live at home.
His friends live a couple of houses down in the cul-de-sac. So by now I pretty much I learned when it happenens just not where ( like tonite maybe ). My brother works swing shift from 7pm to 530 am. So he can't know right?
I think she knows that I know just cause of the way she acts around me though I never asked her about it since she's grown. My brother has never neen the type to listen whenever people try to tell him something
Its just his personality which is probably why I never said anything yet.

Any opinions?

Me and my boyfriend been together for over 1 year
He has never cheated on me..
We see each other mostly everyday
And he treats me good

But today I saw him commenting on this girl's gym selfie on instagram (on the 1st photo)
Apparently they went to the same high school:

The blue is my boyfriend
The red is the girl

And then I confronted her by messaging her directly on instagram and this is what she said:
( on the 2nd and 3rd photos)
And then she unfollowed him immediately

And then I confronted him in person
And he said "I was just complimenting her, she's back to the gym after having kids, I didn't mean to hurt you, I didn't mean to flirt with her, Im sorry"
And then told him to delete her on fb and instagram
And he did instantly