One thing to consider is the age difference between the kids (13/10 vs. 6). Teens and pre-teens are exposed to much more negative stuff and they also have the sibling rivalry thing going on that your daugther (an only?) might not. I'm not trying to defend them in any way for their misbehavior....just looking for factors that might contribute.

Do you think it might help to try to engage the cousins positively in something without the sister-in-law? Bowling or something....spend some time together and reinforce any positive behavior they show?

cj/