I would suggest that you tell your brother that he needs to have a frank discussion with his children about boundaries. He should also get his children into therapy given that the going back and forth between home and foster care can be hard on children. The therapist should be informed about the acting out behavior. He should let them know that they can talk to him about anything.

You should tell your daughter that other people touching her/ taking pictures/ showing others her private parts or anyone showing theirs or having her touch theirs is not okay. That if anyone asks her to do this she should say 'no' and come find you immediately. That she wont be in trouble for telling you. You don't want her to do this, but if she feels like she will get in trouble by telling you, she wont tell you if someone were to do this.

Take a look at this resource: http://www.nspcc.org.uk/inform/publi..._wda48672.html and http://www.rainn.org/get-information...sexual-assault