5 year old nephew fighting with siblings

My 5 year old nephew is becoming out of control.  first a little background.  father recently relocated out of state mom and kids have not moved yet, but spend almost every day and night at gramas house.  nephew is not yelling bad language, disrespectfull, hitting siblings, pinching, pushing, kicking, when mom trys disiplen he laughs and the situation escalated.  mom has tryed time out, taking away privelages, and even spanking.  when he won't behave or go to  bed mom and grama calls me to intervein.  he goes right to bed for me because he knows i will not put up with his behavior.  grama wanted me to take him for a couple of weeks to straighten him out.  i refused, i don't beleive it will solve the problem but rather delay it.  i do not know what to do i have expressed my concerns about them not being home in their beds, and dad not being here.  i give sugestions and mom won't follow threw.  bottem lline is this rebelloin or is their something else that needs attention. any sugestions will be apriciated

[margypops]

You say his mom has not moved yet I think this child should be looked after by his Mom and not handed around to all the family why is it he cannot stay in his own home with his Mom and other siblings ..

[faygram]

him mom and other siblings are all at gramas mom does not want to stay at home i do not know why