Aunt but feel like their mom??

My sister is 23 and I'm 15. She has a 3 almost 4 year old. I get up with the baby every morning and make her breakfast while her mom sleeps in til noon, sometimes closer to 2. We try to wake me sister but she either yells or just rolls over. My nieces dad is barely in her life so it's really just my mom and I taking care of her. My mom puts her to bed when her mom is at work 5 days a week(she cleans banks and leaves at 8 and is home by 1). My niece calls me mom sometimes and has recently told me that she wishes I was her mom. How do I make my sister step up and get off her phone and take care of her kid so that I can be a kid. I don't go out anymore because alana(the niece) cries as soon as I walk out the door.