Need to vent

Ok so I live with my fiancé and his family we have 4 kids between us 3 are mine one is his and we’re trying. Ok a family friend is staying in the home and she is constantly around us in my bed ect she is 19 she keeps trying to put me in my place by saying shit like I knew him longer he doesn’t like that and gets mad when he does she is trying to be mom to the kids and mislead me on what he likes she can’t have a inside voice when speaking to my kids instead yells always when he and I want to go on a date she tags along I’m getting. Fed up and I want to get a apartment before I snap and put her in her place idk how to do so nicely my fiancé needs to stay there to care for his father who has health issues so I get it but what to do about the bratty teen?