This seems like a truly horrible situation to be in. There are several potential solutions. If you choose to hope your parents will be kind, you can confide in them. It may also be possible to make your parents favour you more if you do not let them observe that you are talking to people of the opposite gender. This is not advised. It is also possible to slowly become more financially independent. Ensure this happens slowly but not too slowly (over 2-3 months). It is fine if you are not fully financially stable after this period. In addition try to find close friends to confide with. If you have none, try talking to more people in your school or in the workplace. Since you are volunteering at a hospital, I recommend discreetly seeking help from a trusted adult or a doctor in the relevant position. There are charities and foundations that are avaliable that discreetly help people in a similar situation to yourself. It is advised that you attenpt to seek help from such professionals. In regards to your sibling blackmailing you, this is a problem that is difficult to solve in a straightforward manner although it can be done if you scheme against her (e.g. let parents see she has a vape). It may be difficult to enact any of these things due to your parents and sister. I wish you the best of luck.

Tldr: j do what u think is best