I would certainly have something to say if one of my grandchildren told me to 'Go away.' Yes, it is the parents' fault - they haven't taught her any manners. But that doesn't mean that you have to put up with her rudeness. Correct her - nicely, of course. And if the parents are present, so much the better - it may remind them that they are not bringing up their daughter correctly. And they are not doing her any favours by being so lax. How will she get on in the big world out there, if she is so impolite to other people? Not well, I'm guessing.
I think that manners (table manners included!) should be taught from the very start, then it just becomes the child's normal habit.
If all else fails have a word with your parent (or both). Tell them how her rudeness affects and hurts you ('Go away', no greeting or goodbye ...?) They will be defensive and offended, no doubt, but on thinking about it, I'm sure that they will take your comments on board and do something about it.
Good luck!