The uncle is so selfish and immature. He's an adult but yet still acts like a high-schooler, being indecisive and all. I'm so annoyed by him

@Monky could you recommend some manga like this that includes the 100% cliches you were talking about?

100% clichés.
I wish it had stayed a mystery.

And then from beta he turned into pedohpile.

TL:TR uncle-san is a beta cuck that didn't chase after the girl. Lesson? Don't be a beta.