Hi.
To answer your question, prostitution is generally defined as performing sexual acts for money. That's the dictionary definition, though. What you choose to call yourself or how you choose to think about yourself in relation to the sex acts is another matter.
I am curious to know what prompted you to perform oral sex on your sister's guy friends? Was it your idea or did you experience pressure to do so? Keep in mind that pressure doesn't just mean someone trying to talk you into doing something; it can also mean being in an environment in which going against the grain makes you feel like an outsider, or when doing what others do makes you feel like part of the in-crowd.
I also wonder whether your sister has sexual relationships with the guys she brings home as well, and how old she is.
If you feel that your sexual contact is linked to pressure or perhaps even some prior sexual abuse, I would urge you to talk to a school counselor about this.
No matter what has prompted this situation, if you are performing oral sex on guys without condoms, consider using them. Though there is debate about whether HIV can be transmitted orally (some people have reported contracting the condition from having oral sex alone), we do know for sure that other sexually transmitted diseases can be transmitted this way, so protect yourself.
Take care.
