My ex lets my daughter believe I'm her uncle.?

I have a 5yr old named Bella and her mother and I didn't work out. It wasn't my choice but to the point I ended up having to take her (my ex) to court 10,000 dollars later I am joint managing conservator and I see Bella every other wkend. She has always called me daddy until last wk. My ex and I are both married and everything has been smoothed out over the yrs so to this I didn't want to jump to conclusions., heres how it went down. my daughter told my wife last wk that I was her uncle! My wife said no he's your daddy and she looked confused and asked why she has two daddy's? We didn't know what to do. I felt like an outsider weak and powerless. So to the conundrum I changed the subject. Well in short, the next day my ex called me and said Bella was acting depressed. She said when she asked Bella why she was sad she said "because Sierra said Sam was my "real" daddy." my ex was railing into me about it. I said if she's old enough to ask, perhaps she's old enough to know. And my ex goes off the rails (my ex) "are you ready to have the sex talk? And tell her that we had sex and that's why I'm her dad!?!?" she started talking negative and filthy so I stopped her and said she's called me daddy always and someone has put an idea into her head that I'm something else. She refuses to ask her family and they never liked me. I don't know her husband because he won't interact with me. I dnot know what to do. My little girl is the sun and her mother is the ice that formed in the shadows of our past. Her mother is so mean and spiteful, but I can handle it. My baby girl is more than worth it. I don't want to be demoted to uncle even if it's the best uncle. She looks like me! And reminds me of the things I lost sight of growing up. How do I take back lost ground. Is she old enough to know the truth? minus the sex talk of course.