You might need to draw a diagram to understand this one.
My dad was born when my grandma was 18 and my "grandpa" was 14. He never looked like his "dad" and always thought his mom had an affair (for context, my dad's family is all Lebanese but he is very fair-skinned, which was partially why he assumed it had been an affair). When his "dad's" father, my great-grand-father, was on his death bed due to cancer, a relative confessed to my dad that his "grandfather" was actually his father. My dad had my stepmom take hair out of his real father's head and had it sent for DNA testing which confirmed it (yeah, little morbid if you ask me).
So basically, my grandma had an affair with a married man when she was 18, had one, possibly 2, children with him, then married his SON and had another 4 kids. So my dad's siblings are both his siblings and his nieces/nephews, and the man who raised him is actually his brother.
Yeah, I don't talk to that side of the family anymore.