Need some help and advice with bullies..

...it's not me that has the problem with bullies, but my 12 year old sister. I really want to help her out , she's not a confrontational person at all and will not stand up for herself (unless it's against our other sister who's 20) I'm 24 and definitely too old to do the "I'm the big sister, please back off thing" but that's what this 'friend' is doing with her sister(15/16 years old) unless my sister is friends with her. She is genuinely scared, and continues to be friends with this horrible girl who's leading her down the wrong path. Any advice on anything I can do to help would be really appreciated.