Bratty Little Sister + Impatient Big Sister + Single Mom= ?

My mom's a single mom and that means that i have to take care of my 8-year-old sister most of the time. I usually dont have much time to discipline her because of my homework (im in high school), my chores, and its not like i have a patient nature either. I keep screaming and now she's started screaming back at me. She's really bratty and just doesnt respect me as an elder. I want to be her nice and fun sister but i also dont want to be trampled over by her. Please help!