I wondered why I couldn't post on a thread with a similar title. Seems it had been quickly zapped for breaking talk guidelines.
No idea why as it was just someone looking for advice.
I can't remember the name of the original poster so I'm hoping that this doesn't get deleted and that she manages to read it.
I had copied my original reply so here it is:
I would contact the Bayswater support group for advice.
It's for parents whose children have been caught up in this so I should imagine there will be people who have experience of explaining the situation to younger siblings.
www.bayswatersupport.org.uk/