Why am I getting in trouble with my mom, for being confused?

Yesterday was my older brother's dishes day. He only has ONE day out of the entire week to wash dishes. Last night, he didn't wash them. This morning my mom woke me up, and she said "do your brother's dishes." I was confused because I didn't know why I had to do them instead of my brother doing it. I asked "wait, why isn't he doing them?" And my mom just got mad. She was like "because I told you to do it. I'm SO SORRY for taking time out of your INCREDIBLY BUSY day, to do something as SMALL as washing the dishes. And then she just grabbed her shoes and left for work. This always HAPPENS and im so tired of it. Me and my sister are constantly doing things for my brother, and he NEVER helps us out.