This situation is intolerable. My kids were best friends especially as they got older. And shame on your parents for not putting their foot down. It is not out of the realm of possibility to move out of the house, into a dorm, into a shared apartment. Even if you have to have a part time job to pay rent, that is a lot better than being with this family of yours! Your sister will get more jealous but your parents will be proud. Just do it. Don’t say you can’t. Most young adults do. Don’t ask for help or permission. Just get out of there.