My mom had a baby, but makes me take care of him?

Okay, so I have a 5 month old brother. I also have a 3 year old brother too. My mom only had the baby so that way my brother would have someone to play with, which if you think about it it's kinda messed up. She technically didn't even want the baby for herself. She gives all her attention to the 3 year old, so she barely has time for the baby. Which means i'm always stuck with him. I'm homeschooled, so I'm home all the time. When Im supposed to be doing my work, she gives me the baby. Whenever he has to eat, I feed him. Whenever we go to the store, she makes me take him in the stroller and go to the other side of the store. Everything she should be doing, I'm doing. Don't get me wrong, I love him to death. But if i wanted to be a teen mom, i woulda went out and had my own baby. I understand that Im his sister and need to help, but not THIS much. Especially when she doesn't have a good excuse not to be taking care of him herself. I've talked to her about it, she just told me to shut up. What should I do now? Thank you /: