Evidence is given for the existence of unseen relatives on Jake's side of the family, though we have only actually seen his mother Ruth and heard about his father "Mad Dog".
Ruth makes the statement that she raised her husband's "children" so Jake must have at least one sibling. In addition, a photo in "The Daria Diaries" shows
Quinn, Helen, and
Daria dressed up for a cousin's wedding. Since they don't seem to have any cousins on Helen's side besides
Erin, and since Erin was married on the show, it is reasonable to assume that they were going to the wedding of a cousin on Jake's side.