I dunno. My sibs have been rather good but then they are all grown up. But that must be really hard Rachel.
Well, I would first think to just tell them to f off. But really you could try to talk to them about what they are feeling - "this must be really hard on you hey, do your friends give you a hard time about it, must be hard seeing all the attention our parents give me and not giving so much to you, not really fair. Do you miss having an older brother, that must be hard too" That sort of thing. But if they ever needed any help they should come see you, you are still their big sister and you still love them no matter what, even if they are a little ->-bleeped-<-. I would approach them rather than wait till they attack you first.
What do yo parents think of all this?