why everyone just have siblings that they get compaired with in this story huh? xD

That’s how running themes work.

everyone just have the same trauma

I believe everyone in real life have this experience too, but thanks to the parents, the complex doesn't grow, like, in this manga, it started in their childhood and the parents didn't make a stop to it so the it developed to a complex.
For me, i was the youngest in my family, i had 3 older siblings with age gap of, 14 years, 18 years, 22 years, (22 years of age gap