Kids figure this stuff out pretty well. You need to always tell them the truth, follow through with what you say, be the consistent one.
If it is true that you don't like her family, you can tell the kids you don't like the way they behave. Things they have seen and recognized themselves. Tell them that you won't try to keep them from seeing their cousins if they want to, but that you would prefer if they didn't act like that.
As for what your W says about their future, perhaps you could say to your kids that mommy likes to "think out loud" sometimes. She may just be trying out ideas, so not everything she talks about is going to happen.
As for going to court, think about what the court can actually do. It cannot police your W's language on a continuing basis. Your only reason to bring up her "poisoning the well" in court, is to make sure that you keep joint custody, or to press for sole custody. So, keep records of any event or interaction that shows that she is interfering with your relationship with your children. That's a big no-no which could lead to her losing some custody, or at least keep her from gaining sole custody.
