Were the Peverell brothers and Slytherin related or not? In the books it is mentioned that both were ancestors of Voldemort, but I couldn't find their mutual relation.
It's confirmed that the families are united through Marvolo Gaunt. He is explicitly referred to in the book as a direct descendant of Salazar Slytherin. It is also explicitly stated that the Gaunts trace their lineage to Cadmus Peverell.
Because we know both NAMES died (lineage extinct in the male line, but not dead) and because the Gaunt family is the uniting factor, it can be reasonably deduced that Cadmus Peverell and Salazar Slytherin were not directly related in their own time, but that their descendants intermarried.
The short answer: no.