What country are you living in? That will help with the advice because some services may or may not be available to you.
Call CPS for all of you look for a women’s and childrens shelter and god speed to all 3 of you
i’ve never been in your situation, so i’m not 100% sure but i’d say you should start by finding a homeless shelter to stay at*, preferably far-ish away from your mom and stepdad so they won’t find you easily. and i believe the age you are able to get a job at is 16 so you and your sister could get jobs, so that you have your own income to live off of. you’d have to have someone watch your sister while you work but that would still be a good start i think. feel free to message me if you need any further help, i don’t know where you are but regardless i am happy to help you find anything you need
*assuming you don’t have any other family that would listen + understand the situation and take care of you for now. or maybe a friend’s parents’ would be doable if they were understanding?