I do wanna point out that one of if not both the twins could also be intersex!
You’ve activated the infodump button regarding trans headcanons for the younger Pines twins:
At least one of those kids is trans. They’re said to be identical twins, which means they would’ve been assigned the same sex at birth. In order for them to be presenting as different genders at the time of the show, that means that one is trans. (Or they both are, but one hasn’t figured it out yet.)
I lean towards Dipper being transmasc and Mabel being cis, but I really enjoy all the different variations of trans headcanons people have come up with.
Gonna be a little obsessed with the idea of them both being transmasc since the previous anon mentioned transmasc Mabel, because I’m gonna be real, it hadn’t occurred to me that they might both be trans until reading that ask.
I think Gravity Falls has enough weirdness going on that thanks to supernatural gubbins the two can be trans in every direction and cis all at the same time, like quantum physics.