Hi, I'd like to help you with your question.
It sounds like your mother and sister are fearful of you relapsing and returning to your former addictions. Although their fear is understandable, they do not have a right to take over your prescriptions or contact the pharmacy about your former addiction.
It may be that your family is confusing you living with them as having control over your life, as they did when you were underage. You are correct, you are an adult now and you have your own choices to make. They do not have a right to take anything of yours. And part of recovery is learning to be responsible. This situation is threatening that responsibility.
The only way they could be allowed to take away your medications is if they had power of attorney, like you said.
Try again to talk to your mother and sister. Let them know you appreciate their concern, but that you are a consenting adult and you will take care of yourself and your sobriety. Then set some ground rules. For example, they are not to touch your personal items or come into your room without your permission.
If you are paying any type of rent or contributing in any way to the household, then you are to be treated like any other renter would. If you are not, then consider starting to contribute. It would help your mother separate seeing you from the dependency you had as a child and help her see you as the adult you are now.
You may want to reconsider your living arrangements. If you can, you might want to start saving now to move out. Also, keep your information private from others. Do not tell anyone where you get your medicine or what you take. Keep your medications in your room under lock and key (invest in a small safe if needed).
If you feel at any time that the stress of the situation is starting to affect you, seek extra help either from a counselor or other support.
I hope this has helped you,