I'm on the opposite side of this fence. My daughter goes to Michigan every summer to be with her dad and if she ever acted like that-heads would roll and she is only 7!!! She knows that I expect her to behave the same way she does at home if not better when she is at someone elses house otherwise she will be in trouble with me as well! And I would want her father to call me if it was something way out of hand with her. You in no way shape or form, including your children, should have to leave your own home when the boys are there. You and your husband need to sit them down and explain to them that these are the rules (what ever you and your hubby agree upon) and that if they don't follow them then there will be punishments. It shouldn't matter if it is you that is home with them, they HAVE to respect you, your house and your rules! Period!! If they can't, then your hubby needs to take those 2 weeks off work each year and deal with them on his own. You need to put your foot down and stand your ground cause I'm sure you wouldn't allow your two children to act this way!
Does their mother know how they behave? Are they allowed to behave the same way when they are at home with her? Either way, you need to envolove her in what is going on (step dad or long term boyfriend too if there is one). If they are treating their own 6yr old brother this way, what are they doing to little kids in their own neighborhood? There is no excuse for their behaviour and they are old enough to know right from wrong.
Good luck and I hope your family can pull together to get this taken care of quickly!
S.