Learn English – a word for the uncle’s ex-wife’s daughter (who was his stepdaughter)

"My ex-wife's child" is the proper description for your blood-or-marriage relationship with this child.

If you are back with your ex, you can call the child your step-child. If you adopt them, they are now your son/daughter.

Unlike "Significant other" (SO), there is no terminology yet to name this particular relationship that you can use and expect to be understood exactly.

Having said that, whose business is it that you would need to explain yourself so precisely? If he loves you like a father, and you love him like a son, and the mother goes along with this, describe him as your step-son. If someone questions you, politely steer the conversation the way you want it to go. "As I was saying, it was a great game. Have you been to any games lately?"

I just read of a term called cross siblings. Where you are not legally or biologically related but you share a half sibling. My kids are tech half siblings but we never call them that because they are growing up together with me. But they both have half siblings, and the half siblings have half siblings. And it can make your brain hurt trying to explain it all lol. But because of this I've googled it a few times. And came across a girl in the UK who uses the term cross sister when it comes to the girl she shares half brothers with, I then googled it and have found it has multiple hits. I don't know how legit the term is but sure is easier to say than brothers half sister or half brothers half sister.lol