Betrayal or Overreacting

Many years ago my ex bestie that I've known since kindergarten who also happened to start a casual relationship with a family member of mine decided sleeping with my family wasn't enough and slept with my childs father while we were together. When i found out we weren't friends after my childs father confessed she denied it n never apologized EVER n i haven't seen or heard from her since. I moved out of that state and i live with the same family member she was sleepin with he tells me shes coming to town and there gonna get together as tho he doesn't kno how bad she hurt me n everyone feels it was so long ago i should let it fam doesn't care about my feelings cuz hes just worried about sleeping with her...but i feel like he's not loyal n i feel betrayed to the tenth power.....OPINIONS PLEEEEASE