He’s not OP’s stepdad though, he’s her dad.
Seriously this doesn’t need to be some kind of sit down talk where you confide some dark secret in your children.
I think I was about 7 when I discovered that my dad’s dad wasn’t his real dad. It was just mentioned in discussion, I don’t even think that I was told specifically, someone just made reference to it.
My uncle though was adopted and I wasn’t not told, but i was told not to just blurt it out as he was actually my cousin and my gran had adopted him. He, on the other hand, was told by a child at school when he was five that “your mum’s not your real mum,” presumably from a child who had overheard their parents gossiping.
But this isn’t the same as a child being told by their adoptive parents that they were adopted, people make far too much of this biological link further down the family, and the the almost expectation that extended family should be upset and feel betrayed at being told. Why? While your dad didn’t come into your life until you were three, their grandad has always been their grandad. They absolutely don’t need to know, but there’s nothing wrong with them knowing but it doesn’t need to be made a thing of.
I mean in reality how many of us are interested in our grandparents’ extended families? Depending on the ages likely not that much. Kids don’t have nearly the kind of connection to great aunts and uncles and second cousins as parents have to their aunts and cousins.
Question is, is your dad your dad? Or do you feel some kind of sense of loss and wish he wasn’t? Is he a good father? Because that’s what counts.