Board Thread:General Discussion/@comment-4115284-20150428133857/@comment-12265996-20150614205151

BoomCipher wrote: There are only two choices before us. Stan is Gruncle. Grand Uncle. That means that either the Author is Dipper and Mabel's Grandfather, or there is another Skywalker.... Sibling! I meant sibling!!

And he said "my brother" which implies that the sibling is a woman. Had he said "Your Grandfather" I think Mabel would have fainted! But the AMA does make it sound like it's a brother...

It has to be a brother, for Dipper and Mabel to have the Pines last name