How should I feel about my "younger sister" being in a relationship with my ex?

To explain, I have a close friend, like a sister to me that recently started dating one of my exes. She is intimate with a guy who has been down on me and I have been down on. We never slept together but I'm still not sure how I feel about this or why it's bugging me. Any thoughts here might help.
Updates
+1 y
Since it's come up, when I was dating him I did things that she doesn't know about and I don't want her to think bad of me and I don't know if he's still does those things or if he's grown like I did. I'm worried for her but I just don't know how to ask him or talk to her about stuff without telling her what all I did and now am ashamed of.
Updates
+1 y
Update to this, they broke up, he was more forceful, (what I was worried about) it frieked her out and scared her. Thankfully, hubby and I were able to convince him that her decision to end the relationship was final as long as she so chooses. He always was a little possessive, he tried the same shit with me. I took martial arts for years and convinced him not to try to force me to change my mind. I went along with some things that I really shouldn't. I'm glad she is safe
How should I feel about my "younger sister" being in a relationship with my ex?
Post Opinion