WIBTA for not taking care of my baby brother?

My father has me (17) and my baby brother (1) with my step mom
For context i live with them bc its closer from my school so i almost doesnt see my mother

So, basically i was watching tv, father working, step mom taking care of baby brother (she also works but less than my father)
She called my father to help her bc baby brother didnt want to eat, i went to tell him she was calling him, he said to go help there
I went there, helped to feed my brother (didn’t take long)
Some minutes later she called him again to help, he grabbed my brother and handled to me, I accepted, but he wanted to turn off the tv bc it would agitate the bb, I complained and said he didn’t need to shut down the tv and I could take care of my brother while watching it, he got mad, grabbed my bb brother and gave him to my step mom again
Then she went to talk with him, i didnt pay attention bc of tv
He started playing with my brother in his home office after that

If I say I have no obligation of taking care of the baby he choose to have will I be an asshole?
I have no problem at all taking care of him, I would turn off the tv and help, but he cursing at me like its my chore to take care of him annoyed me so much that i felt like not doing it just to annoy him
He works a lot and for me and our family for our own good and i was doing nothing but watching the tv, if I helped a bit it would be less stressful for both of them
But at the same time I hate when he gets mad bc im not helping something that has nothing to do with me and he choose to do
If he asked nicely i would have took care of my brother without complainants
But also I don’t think I should not take care of him bc I’m mad with my father, he got nothing to do with us
Help?