Younger siblings help

Right now I don’t know how to deal with my younger sister (note that my mom doesn’t not deal with “parenting duties”well and we have different dads who are not physically present.) my sister is driving me insane today I ask her to watch over my tofu on the skillet and turn off the stove if it starts to burn because I had to shower and she is old enough to JUST watch over it and not injure herself(she’s 11) Now I cut it in cubes and when I come out of the shower to check on her and my tofu tell me why it is mashed up in the pan and in crumbles brunt. I ask her what did you do to my tofu and she says didn’t do anything and that it was already like that. Now me and her both know that is a lie because it is only us in the house so no one else could have touched it. I ask her why is she lying when we both know that is a lie and just goes on to shrug her shoulders and roll her eyes. This isn’t the only incident of her behavior and I know she is 11 so ofc there is going to be so much more of these shenanigans and attitude but she has been this stubborn and mean her whole and life and has maturing issues like for example she still sleeps in our mom’s bed. Another example of this attitude another night it was 2ish in the morning and I went to go get some water until I that see all my mom lights were on in her room and she was asleep so I turned them off. So I go get my water head back to my room when I see my her lights were turning on again so I go check and it is my sister turning them on. Like are you serious your mom is clearly trying to sleep and I ask her what is she doing and she just says “ Just leave me alone” Like what?!?! Anyways what should I do so she does turn out to be a sucky person when she is older because she just doesn’t treat me like this and I’m genuinely concerned unfortunately our mom is not exactly helping out.
(Sorry for grammatical errors)