Sounds like your sister had been pampered a bit too much? Yea,
definitely.
Actually, because you or your parents give in to her easily as so
to stop her tantrums, she became used to it and she thinks that
whenever she can't get something, she just have to throw a tantrum
and TA-DA, she gets it.
And because of this, she actually continue doing it.
Firstly, I don't think that your sister verbally
abusing your brother is legal. If I recall from a newspaper article
from LONG LONG AGO, it is illegal and your sister can be sent to
the Girls Home if she is 21 and below.
A question: Do you mean that whenever someone visits, like your
grandparents/relatives/friends..., she behave very well?
Well, if it is yes, all the more you should ask them to visit.
Especially grandparents as children are usually meek in front of
them.
There could also be some sibling jealousy between
your little brother and her as she picks on him everytime. She
could be also going through mood swings, even though mood swings
should be gone at the age of 16. She might be going through it
longer. Also, I think that seeing the younger brother getting the
better things make her feel that she is not loved the MOST as
usually, girls are attention-seekers. Not all girls though. And the
fact that YOUR brother gives in is also another issue you need to
look at. You and your family members excluding your sister MUST
tell him or advise him to be STRONG and CONFIDENT and don't be so
scared. If the sister yells at him, yell back. If the sister hits
you, tell them. Eventually, I think she have enough of it. But if
she still continues, maybe you can ignore her totally and pretend
to shower your brother with more attention and love. 
Last, if she is STILL the same, you have to, and
MUST, consult a consulter. Best of lucks to you changing your
sister to a whole different person!! May SUCESS be yours.
(P.S. Oh, and one thing. You must try to be patient! If you lose
your patience, it isn't good. Oh yea, it isn't good. Not even a
bit.)
