Sometimes I wonder if Alex Hirsch did this on purpose as a joke to the fans... He did have them eating at a mexican restaurant in the "Golf Wars" episode on purpose; making fun of "Dipper goes to Taco Bell"... This guy literally does not care what people say and he puts in the subtle jokes that are inappropriate to give the fans a good laugh. (He knows what happens on the internet...)
I just see this as normal. Holding hands don't mean instant incest. Heck, I've seen my classmates who were twins, in different gender and yet I saw them hold hands and then talking random stuff I forgot about.
It's a normal action by anyone. Don't ship them the instant they do something the shipped victims do.