I'm not going to make my sister a burrito in the microwave
What would you do if your sister wanted to microwave a burrito?
I would probably ask her what the fuck she was doing there.
Your sister shouldn't need you to microwave things for her if she is fully capable of it herself. Not bending to her every whim will make it so that she doesn't become lazy.
She needs to start making her own food and leave me the f*** alone.
She was probably there to make the burrito herself since you refused. It's good to encourage her to be self-reliant.