Why are older siblings so rude and mean to younger siblings?

My adult siblings are always hard on me. They get on me for not listening to dad, call me out for being a brat and called me out for me using my disability as an excuse and I’m tired of them pointing out about me.
Like say if I’m rude to them, they’re rude back. One time my sister treats me so meanly. I was being a rude brat and she treated me the same way. I cried to her to stop. Then she sternly said, “Are you going to start being respectful then?!”
I said yes and I hate that she’s mean.
She told me, “Well stop being mean to people then. I was mean to you because I wanted you to see how it felt to be treated that way. Respect your elders.” And she shoved me. Meanie!!
I told my bro and he said, “Well treat others the way YOU wanna be treated then. If you’re going to be ugly, then people will react to you the same way.”
I’m very polite now but do push people buttons people get annoyed with me when I tell them to be nice all the time.