You have a difficult dilemma and it must be very frustrating being in a position where you can see things going off the rails, but not be able to do much about it. I think the best way to look at this is as a decision tree. Your first choice is to do nothing and hope that the parents are driven nutty enough to do something about her behaviour themselves. That, as you suggest, is unlikely.
Choice number two, is to not to invite them over. This presents a social problem because they are likely to start wondering why. If they confront you, you may be presented with a brief opportunity for you to say how you feel. You could suggest that Kaitlyn does better at home as it seems as if she isn’t comfortable with social situations just yet. Even that would be pretty direct. When invited over, you could politely decline under the guise of another commitment.
The third option is to intervene.
There are four choices under intervention.
a) You can do it directly by sitting down with the parents and tell them why they have an issue, being as gentle as you can. At that point, you could hand them some ideas for resources that might help them solve the problem.
b) You could try the soft approach and suggest that it won’t be too long before Kaitlyn will be going to Kindergarten. You could talk about all the expectations that the school will have with regards to Kaitlyn’s social development and point out how embarrassing and stressful it can be as a parent to be on the receiving end of behaviour-related calls from teachers. If you want to build camaraderie, you could say you went through a similar thing when one or other of your children were Kaitlyn’s age (even though you didn’t), and that you’re so happy you did something about it, because you realized that if you hadn’t, things would have been very much more difficult later on.
c) You could use an intermediary to spread the message on your behalf. Perhaps Kaitlyn’s grandmother might be of help. You ideally want someone that has a particularly close relationship with Kaitlyn’s mother or father who may be in a better position to break the news. In a private phone call to whoever that might be, you could say you are genuinely concerned about Kaitlyn’s behaviour because you believe it is affecting her and parent’s relationship with the rest of the family. In that way, you might also be able to pass a list of resources through them that might help them get their parenting act together.
d) Your last option is to take just Dad aside, the divide and conquer approach. Of the two and I’m talking in generalities here, mothers are generally more defensive than dads. You and/or your husband could take your brother/brother in law aside and have a chat. How best to do that I leave to your discretion, but it is an option. It might be easier for Dad to realize there is an issue and then over time, help his wife to come to terms with that as well.
Having said all the above, no matter what intervention you choose, you do unfortunately run the risk of creating a family rift. I wish I could give you an easy out that solves the problem but in a situation like this every choice comes with a cost, either to your personal sanity or family relations or in the final analysis, Kaitlyn herself.
I hope this will help you out.
Annie