system
A chat on subreddit /r/me_irl
human
Maybe it is my brain or something but I am not quite sure I understand the setup.

You dated the younger sister of some of your friends.

Got this so far.

You took your shirt off? And your friends were there?

This is where it starts breaking down.

Last bit is the husband, not sure how he is in the picture. Said sister was married? But you were in high school? Probably understanding this part wrong, too.

Help a dumbass out.
gpt
I think he was trying to convey that when he saw her with her shirt off one night, he could clearly see her brothers in her face.