First of all, i want to tell you that i really feel for you and understand your nervousness about you relatives finding out.i know that its really embarrassing and can only imagine how i would feel if the same thing happened to me.However like mr. emunah said this might be good for you.
Second,maybe you should talk to whoever is going to find out before they even get the email and explain the problem that you have, and ask them not to tell anybody about it (unless there would be a toeles). if you explain the problem before they find out then you can present it to them the way you want to instead of them finding out and thinking all sorts of things before you speak to them. first impressions help or hurt alot. another benefit oof doing this now is that maybe after you tell your sister about the problem itll also be easier to study trust me a confession feels good afterwards.
Third, you ended off saying that you fear that you will still be an addict, whether your're an addict or not is not something to fear the problem is there and this is not going to change it. if you are an addict then accept it (its not your fault its a sickness) and work accordingly and if you aren't an addict you still have a problem that you have to deal with and with hashem's help you will
also trying to find loopholes to fix might not be the best idea. this reminds me of years ago when i decided to go onto a p*** website and shut the browser to show hashem how i'm not going to fall even if im in the same situation you can guess how that turned out...
anyway good luck and we're all here for you