I honestly do not know what to say...i know exactly how you feel, but i was never good at expressing things either.
Well, just like the person before me said - try to tell your parents what they have done. Try to tell your sister too. E-mail them a webpage if you are too nervous. Maybe they love you more than you think they do, maybe they'll take you to therapy and try to correct their mistakes as well. Who knows what could happen?
If not... well, then why should you give a crap about them if they don't give a crap about you? Just ignore them and focus on you for a while. Slowly try and make some new friends at school. Every week, try to talk to somebody new. Then, a month or two from now, take it to the next step and get some emails or phone numbers and call them - try to talk to them for 5 minutes. Then, in a couple of weeks, try 10. Then, in a month, ask them to go out to the movies or something with you.
No matter how shy or nervous you may feel - don't cancel. This is actually really what therapy is - it's just slowly facing your fears, because facing your fears is really the only way to get over them.
So try your best to mingle. And know who your real friends are. You can always rely on them for support. Your boyfriend may be the best tool for that if you are serious. Just remember that whatever happens will happen. Slowly open yourself to him as best you can and see how he reacts. If he leaves you, obviously he wasn't the right one for you. You'll find someone better. If he stays - you've got something i've never experienced.