How far can being mean really go?

My sister Gabrielle hates my friend Julianna SO much. Everytime she comes over, Gaby picks on her, insults her, and makes her want to go home. I hate it! But now, I think she's gone too far. The first of the bad happenings started when Julianna dropped off a form that she made for people in our neighborhood, explaining how she would love to babysit for those who went away. Gaby was with us and spoke directly to me, even though Julianna was right there. she had said "Why should we hire her? She can't even take care of her own skin!" and i thought Julianna was going to cry. Also, Gabrielle blackmailed Julianna and ruined her school year, as well as any future school years. I feel like I can't have any friends because of my stupid sister. Is this all going too far? How to stop it?