i think i saw a different one but they didnt have sex i think it go like this:
dipper reading a book and mabel shows up and bites dippers nipple and dipper scream and it cut for some reason
and mable was like relaxing and dipper shows up with an angry face and crying i bit and then he bited mabels nipple and mable moans and dipper and mabel was both was so angry

Incest is life

if only incest was actually as good as this in real life :(

Incest >_<

Incest is wincest