The relationship between the Youngers and John Peters Ringo was by marriage only, and not by blood.
Ringo's mother, Mary Peters Ringo, was the sister of Col. Coleman Younger's wife, Augusta Peters Inskeep Younger. Coleman Younger (for whom his more famous nephew was named) was the brother of the Younger boy's father. His sister (or half-sister -- it's hard to keep the relationships of all 19 of their father's children straight), Adeline, married Lewis Dalton and was the mother of the outlaw Dalton boys. Another brother, Bruce, was married for a time to the woman who later became known as Belle Starr.
Taking it back a bit further, Ringo's maternal grandmother was the sister of Benjamin Simms -- the second husband of Zerelda James and the stepfather of Frank & Jesse.
Just imagine the potential family reunion!