singlemumma, the same thing happened to my Nana's eldest brother.
They grew up with 'Uncle Willy' who lived in the woodshed out the back of their grandparents house. The story only came out after their parents and Willy had passed away.
I've done extensive genealogy, in this particular branch the struggle is matching the right people with the right parents as it seemed in every generation almost every child had a child out of wedlock who they gave to their parents or older siblings.
DHs eldest niece (21) has been raised as his little sister and another niece was given to a cousin to raise. His aunt who DD is named for had a son out of wedlock and never revealed the father, she still hasn't, the son was raised by his grandparents and he passed away at 21. Many of MILs siblings are actually her nieces and nephews, she was the youngest biological child and is close in age with these niece/nephew/siblings.