Because Amelia dug herself into the awkward hole with the cousin rejection and now she has to be the first to reach out to Uto. Which is not in her character. It's clear Uto would love to play with Ame. And Nabi already said she's fine with a family collab, just that Uto and Ame don't get along.
There is nothing forcing siblings to play together. Though it is common, because it's an easy way to please fans and have fresh interactions. Ollie played with siblings. So did Moona. And today so does Uto, without even needing anyone to prod her, or worrying about Mana leeching her fanbase. It is just a matter of people wanting to play together, and an understanding that for vtubers, the rising tide lifts all.
Uto would probably be the easiest JP girl for Ame to collab with (besides fluent people like Ayamy), great at both apex and minecraft, and she is like a meme-creating machine with her Utolish. It is simply a lack of will from Ame to be the first to reach out, due to busyness and awkwardness.
