Your situation is fucked, and you let it become fucked. Now that it's incredibly fucked beyond repair you want to just say it's too late to go back now.

Did you ever encourage your brother to see his kids. Stand by him through what was certainly a tough period in his life, getting divorced. Ya if your brother was a better dad he shouldn't have needed that encouragement, but sometimes people fall short of doing the right thing, and family should be there to correct them.

And making out with your brothers ex-wife, that's low. Look at the timeline, within a year of his divorce you were banging his ex-wife, within two years you were married. I'm gonna guess that someone who moves on that quickly to his brother's ex, wasn't there to support his brother.

And then you say I didn't intend to become a dad. But then you don't let the kids call you Daddy. They didn't just start it randomly, you encouraged and fostered it. It's really not that hard to correct them and say "no I'm Uncle". And now that they're 6 you've allowed them to believe this lie. If this is the route you were going to go down you should have contacted your brother and asked him to terminate his parental rights years ago. why didt you do that? Were you afraid this might be the push he needed to step up to the plate and become a decent father and see his kids more often - thus ruining your happily ever after?

If these kids weren't biologically yours it would be easy to say your actions are kinda fucked. But because they are biologically yours emotion makes us feel differently imo. But courts aren't suppose to judge on that emotion - legally those are his kids. When you signed up to be a donor, you signed up to be an Uncle and nothing more. Your asking people how you can prevent a Dad, who has finally got his shit together (with no help from his brother) from seeing my his kids. That's what this boils down to. Sounds pretty crappy when you sa it like that.