User blog comment:PurplePenguin/The Grunkle Stan Mystery/@comment-15628758-20130816122127/@comment-24128378-20131103235340

Stan is Mabel and Dipper's great uncle, meaning he is the brother of their paternal grandfather (since they share the same last name and such). He had a brother. Whether or not his brother was his TWIN is still a debate until made cannon.