Out of all the things you could've done inside your own room, you chose to go to the other treehouse.
You step outside.
Your treehouse lies atop two combined trees, combined in that they merge branches and stuff. You have a rope ladder to the Golden Feild that seas the land between your family's birdhouses.
In your family, when a bird is old enough to leave the main house, they can choose to build a treehouse on top of a tree that was planted when they were laid (as an egg). You and your sister have the unfortunate side-effect of being born in the Underground while it was sealed, and thusly, couldn't take part in your family's ancient traditions.
Your sister and you, after the underground was unsealed, took home in your great great great grandmother's tree as her tree was the most recently planted, and subsequently isn't too entirely overgrown to live in. Your current residence is on a tree which a human planted, almost eerily around your mother's birth. You're not sure where that human is, or who they even are, but you're sure that they were one of the kinder ones.
Your rope bridge to your sister's house is very long and quite dangerous. You better look ahead before you go. So
you do
Your family's house.