AITA for telling my MIL that I cannot believe how classless she is?

My mom died 4 years ago. My dad is 52 will be 53 next month.<br><br>He remarried this woman, Sandra who is 43. She has 2 twin girls that are 13. They moved in with my dad when they got married 2 years ago. I have 3 siblings. We all live nearby with our spouses and children.<br><br>Sandra has never been this trashy mother like she didn’t care for her daughters and made it very apparent. My dad spoiled her girls so they didn’t see it but other than that she wasn’t into her girls.<br><br>It was no surprise when they got sent to a rich boarding school. My dad was thrilled his girls were going to such a nice school. She was happy her girls were gone. Now she only sees them every other weekend.<br><br>My dad and Sandra bought her daughters brand new cars. Nothing is too good for daddy’s girls. They go on a trip every summer with her parents.<br><br>Sandra and my dad come home pretty late at night. They don’t tell us where they were.<br><br>I did some snooping after they went to bed one night when I came over to pick up my niece. I got her diary and read it on my way home.<br><br>Sandra has been cheating on my dad this whole time with some rich guy. It’s been going on for years. He buys her stuff, pays for stuff. The diary goes into detail about how spoiled she is.<br><br>His name is listed. I looked him up and holy shit he is rich like a billionaire.<br><br>I felt like I had to tell my husband what I found out. He said I have to tell my dad but I didn’t want him to know I read her diary.<br><br>I was telling my sister about it yesterday at lunch and my dad overheard us. I didn’t realize he was there. He asked us what we were talking about and I had to tell him.<br><br>He did not take it well and he didn’t believed until I grabbed the diary and handed it to him. He called Sandra at work at the time and told her he needed to talk to her.<br><br>She denied it. So did her lover boy. My dad pressed on more and more until she got fed up and told my dad she was leaving and she wanted a divorce.<br><br>Then all these other things came flying out. My dad was in tears as she yelled at him about how she didn’t love him. How spoiled her girls are. How my dad can’t satisfy her. How her lover is so much better. He buys her everything and gets her anything she wants. How my dad is old and she wanted a younger man. How she doesn’t see why my dad is still crying. Then she told my dad that her lover is the one paying for the boarding school not him.<br><br>My dad cried even harder. He keeps saying that he did everything for her. He gave her girls the world. He gave her the world. He can’t understand that she could be so cruel.<br><br>Her lover boy came and picked her up. My dad is devastated. My siblings and I are just glad that she is gone.<br><br>We told him we were sorry we didn’t tell you sooner. Then my sister said lookout here Sandra comes now. I turned to see her in the doorway with her lover and her daughters.<br><br>I don’t know what possessed me to say this but I said “Wow, you’re back but why? Did you realize that you left your classless ass here?” My dad and siblings were in shock but the girls started crying and her lover boy got mad and asked if I was talking to him like that. Sandra told him it was okay because I was jealous of her.<br><br>AITA?<br><br>Edit: Everyone is asking what age my half sisters are. They are both 13