AITA for telling my former stepdaughter that her father is dying?

I (f) split up with my ex (m) two years ago. He's the father of a teenage daughter (D). D is very clingy and requires a great deal of validation and affirmation. She's very sweet and I loved her like a daughter, but that also meant it was impossible to be alone for even a minute because she had to be next to me all the time. She'd follow me to the bathroom, sit next to me while I read a book, etc. I didn't have a problem with it before, but when I started getting close to my ex, it got a bit awkward. I'd be having a conversation with my ex and she'd interrupt and ask if she could help with anything, or if we're going out to eat, she'd always ask if she could come too. I'd sometimes tell her that it was just going to be me and her dad, and she'd look heartbroken. She did grow up on me because her mother died in a car crash when she was 3. My ex says she was always like this, even as a baby. She'd wail for hours unless she was being held, she'd wake up in the middle of the night and throw a fit if she realised he wasn't in the bed, etc. I obviously took the break up hard, and it meant that the extra hours I could spend with her made me feel better. <br><br>I loved her like a daughter, but when my son was born, D started acting out. She threw a massive tantrum at my son's birthday party (he's 2) and accused me of no longer caring about her because she wasn't "my real daughter". I was very taken aback because I'd never seen her behave like that.<br><br>My ex says she's been doing that a lot ever since. She's always acting out and he's at his wits end. Apparently she threw a huge fit at his office because she found out he was in a relationship with someone else, and she accuses him of not caring about her any more. That's why I haven't seen her in over a year. <br><br>I told my ex that I'd be happy to see her again if she could behave herself and respect my boundaries. I don't want her to think she can throw a fit at my house and get away with it.<br><br>Anyway. My ex was diagnosed with a terminal brain tumour. He doesn't have long left. He isn't telling D because he doesn't want her to throw a fit and make the rest of his life a living hell. <br><br>I told him I disagreed with that and he asked me why. I told him that she's an adult now and deserves to know the truth. He said she's not an adult and she's still his "little girl" and she'll always be his baby and no one can tell him how to be a good father to her and I got so fed up I just told her. I told her while she was at university and she called him as soon as she found out and is now refusing to leave his side. <br><br>AITA for telling her?