Can Josh talk to her and find out what the big deal is? I don't know if their parents are still living, but if it's been any kind of the two of them 'against the world' sort of thing, she might be afraid and very jealous.

May you and Josh can both sit down with her. She might try to ignore you or be 'too busy for lunch', but she can't ignore her brother. I rather think it is her family's job to help with this.