Let your DD know that first off, real friends will not act like that. If it was me, I would tell my daughter to ignore someone who treats her like that. As far as her bullying her little sis, she is having a hard time, but it's not excusable.
I would just sit her down and explain to her that she doesn't want to be friends with that girl, because that girl is not a real friend, and that she still needs to treat her sis with love, the way she wants to be treated.
I would just sit her down and explain to her that she doesn't want to be friends with that girl, because that girl is not a real friend, and that she still needs to treat her sis with love, the way she wants to be treated.