AITA for telling my MIL that if she doesn't delete her post, I'll tell everyone the real reason I broke up with her daughter.

2 months ago, I broke up with my boyfriend. He's the brother of my ex-fiancé, who sadly passed away 3 years ago. I was in shock for over a year after his death, and didn't start dating his brother until 18 months later. Neither myself or my ex-fiancé's family knew I was dating him as we kept it a secret, but when we broke up, they were livid and blamed me for breaking his heart as they thought of me as a daughter. <br><br>I would regularly get messages from them, which annoyed me but I just ignored it. However, last week, my ex-fiancé's mother posted on social media that she's disgusted and upset that I'm dating someone else. I got messages from people I didn't even know who were giving me shit for it, and it upset me enough that I sent her a message saying if she doesn't delete that post, I'll tell everyone the real reason I broke up with her son. She asked what I meant and I said I'd just tell everyone the truth. She's not replied since and I'm getting anxious. I don't even care if I get shit for this, I just want her to get off my back and leave me alone, as the constant messages and comments is getting too much.<br><br>EDIT: the reason I broke up with him is that when we got engaged, we made a decision not to be sexually active until we were married. However, we used to do other things, but on Valentine's day, he came in my mouth and then got me pregnant with our beautiful 3 children, Ethan, Lily, Ava and our baby on the way, and we then consummated our relationship when I was 5 months pregnant. However, he had a few secrets, namely, that he already had a kid of his own, and one on the way with another woman. I felt disgusted and betrayed. I even considered abortion, but in the end, I cut all ties with him and I'm glad I did. I even have a different last name for them, as I don't want them having any association with him. I blocked him and his entire family on all my social media accounts, and have had no contact with them since.<br><br>I also had no idea his mother was my follower until she posted that, but I do have public accounts, so it's not my fault.