This is pretty common, I believe....I have a "half sister" also...only saw her once, many years ago. My father had so many "girl friends". And my true son admitted he was the father of a child in another state, (that I've never seen).
And another "shocker" a few years ago. This lady, who was a friend of the family, told me that her son (now age 16) was my grandson!...A DNA has never been done yet, tho'. So I'm pretty skeptical about it all. When I did meet the boy for the first time...I noticed many things that reminded me of my true son.
I'm sure there are many, many other cases out there.
