You are finding out the bad part of having family. There usually is better stuff that go along with the bad stuff.
Remember that some day you will be old and your kid's spouses will think you are an a$$hole too. It kind of goes with the territory.
As far as the flea issue. That is a health issue that effects their grand child and daughter.
I guess I am not a real good people person. I usually don't let things fester. I get right to the point and speak my mind. Letting things turn into misunderstandings is worse than a confrontation.