Since this could be a real situation.... maybe... I will treat it as thus look at this http://www.psychologyinfo.com/treatment/childtherapy.html check if she needs therapy because the lack of boundaries issue could mean that she has been abused behind your back.... so try and talk to her about it... or she is just not understanding the reality of the situation so tell your parents/ an adult that she respects or a close friend of hers that can help explain how that isn't really something you do... especially to a family member.