Okay then, uh… let’s start simple. Childhood friends Nuts and Dolts.
Okay, okay, this I can do because I doubt i'll ever draw it:
So, at 13, Penny comes down to Vale from Atlas for a student exchange program at Signal Academy (treating this as one of those K-12 buildings) and ends up in the same class as Ruby.
Ruby's a little weirdo and as much as she's awkward, she's still kind of a social butterfly because she's really good at making friends, and Penny is uncomfortable because, hey new place, and something similar to how they meet in the beginning happens on the hallways.
Now, they'd be glued to each other's hips if they lived in the same area but they're not so they visit each other a lot. Ruby visits Atlas *maybe* once but Penny comes down when she can, which is normally during breaks. But they message and video nonstop. Like facetiming while one's cooking or cleaning and the other's doing homework or playing games.
Yang meets Penny the first time and clocks her immediately, and definitely adopts her as second sister.