Look, how do u think u would have felt if u were the one who
were "given away", and put to stay with ur grandparent instead?
Then for so many years, only because ur brother/sister is going
somewhere else liao, then ask u come back to stay.
My guess would be that she felt that ur parents do not really care
about her in the first place, and that they were biased against
her, and making her stay with someone else rather than with u
people. As u people grow together as a family, it seems like she is
now an outsider. Of course she will feel that ur parents do not
love her. Plus, her being forced to break up with her bf only makes
matters worst.
Try talking to her, and tell her that even though she never stays
with u people, all of u love her. That's what she needs, not
scolding or a slap.