How do I stop my sister being isolated by what is the entire sixth form?

Ignore them. The reason they are continuously isolating your sister is because they know she doesn't like it. The "it girls" know how to effect your sister, they know how to make her feel small and they know her in general because at some point, all of them were friends. Your sister needs to make it clear that she isn't going to let them run amuck with her life. Allow these doe doe's to set themselves up for their comeuppance (karma, "what goes around comes around"). Leave them to wallow in their own immaturity. Set your sister up with some new friends. People whom you trust, and people you know will treat her right. I ume she has little confidence at the moment, so I would suggest that you and your sister leave the year 6 / 6thform out of the equation. They DO NOT matter. Once you have one another, you'll get by easily.