AITAH for refusing to help my parents care for my son who they adopted.

I am an oops baby. My folks had me when they were in their early forties. I got pregnant when I was in university. I probably can’t write here why I didn’t want it. I wanted to not be pregnant but my parents said they would cut off all aid to me if I did that. So I carried it to term. Then I wanted to put the child up for adoption or drop him at a fire station or something. I was not on a good place.
My parents insisted on adopting him so he is legally my brother. I do not hate him but he is a reminder of something terrible that happened to me. I now go back home as little as possible. Maybe two days a year. I have graduated and I have a life far away from my parents.
My “brother is twelve now. And he is not well behaved. My parents are now in their seventies and ar having a shitty retirement. They have to dedicate all their time to him. They can’t take the vacations they planned. They won’t downsize to a condo so they still have yardwork and stuff that he will not help with. None of my actual siblings will watch him to give them a break.
They reached out to me for help. I said no. I said that he is their son and their responsibility. They think I’m being cruel for forcing them to do everything after they helped me so much. It took everything I had in me not to curse them out.
Some of my relatives have reached out to me to see why I refuse to help fix the mess I made. Them I curse out and block. None of them helped me when I was pregnant against my will.
The biological father is on the registry and cannot be near kids. Before you ask.