My Daughter has researched my side of the family and some of my Husbands also. We were able to confirm some by military records. Birth, death records and census help confirm some things. My memory has helped too. My dad's side is harder to find.
My Husband was adopted by his grandparents. He was in constant contact with his birth mother growing up and still today. Legally his mother is his sister, his late aunt was too. He also knew his dad and doesn't have real good memories of him, he is dead now.
He has second cousins who have found sisters and a brother recently, same mother all had different fathers.
I hope you get to meet your cousin soon.