All your sister needs now is love. Love from you and your
parents. In many ways I don't blame her for behaving like that.
Sadly, to her your family is like a stranger since she grows up at
your grandparents' place. What makes you think that you can tell
her what to do (to break up with her bf even though it's with good
intention) etc when all these years none of you show any interest
in her.
While I understand that your parents' predicament to have her stay
with your grandparents, however for someone who is still young like
your sister, all she feels is rejection from your parents. Also,
she has probably been wondering why your parents chose to "abandon"
her instead of you and hence her comments about your parents being
bias.
I think you and your parents need to be patient and try to spend
more time with her to allow all of you to "get to know" each other
as family. Perhaps, your parents should try to explain to her about
the situation and why they made her stay with the grandparents.
Also, she might need to have some counselling, just to help her to
deal with her hurts, resentments etc.
Love her, be supportive to her is all you can do now. Hopefully as
time passes, she will warm up towards your family and will come to
know how much your family loves her.
Good luck!
