This is strictly a vent.
What kind of relationship do you have with your brother?
My brother used to be a slob.
I don't think you need to address this situation until next year (or next summer if you visit him twice a year). Perhaps by then, there will be guy in your life and that will be a non issue. If there is no guy in your life by then, then don't lie about it. I like the idea of helping him clean, even if that means going "behind" him and cleaning up after he cleans something. My brother can be pretty slobby too. I usually don't like staying with him and avoid it at all costs. I do tell him his lack of cleanliness is the reason why.
As far as the bratty kids, I agree with Sadie. No parent wants to hear their kids are brats and telling them so, never seems to improve the situation.
I stayed with my brother earlier this year for a couple of weeks (I was between homes).
All men are like that, regard cleanings.
Is he a younger brother or an older brother? We all know more or less how our relatives act and react.
IMPO, I´d say something about the girl in a very loving supporting way, like "Listen bro; I know some divorced people have issues with guilt and______________________________but have you noticed that your daughter_________________. I say so, because I love you and I love her and
I have a friend who stays at a hotel when he goes to visit his children b/c the grandchildren are noisey and rude and his DD is a single mom with a dirty house.