I had lost my child for A few yrs. the best advise I have when dealing with children and up rooting thier world is to see if family therapy is a possibility. children seem to do good with t's specialized with children{play therapist}seem to earn kids trust. A good one will speak to all parties sererate for while before joinin parent and child sessions together. if this is not A possibility plz try to remember that your son has only really known you sister for comfort and if he not ready for this big of A move it could be harmful to him to have his world turned up side down. the key is he needs to find A outside person that he can talk to about how this all makes him feel and what he wants and expects. as for the rule changes I guess depends on the differences ...has your son turned into A good nice respectful young boy with your sisters rules? if he is A good kid and headed in right direction you should consider if your rules,new to him,is the way to go. just remember hes got A full plate choose your battles