To somewhat balance you your experience, I have been coming to Cambodia for decades and now live in Siem Reap, Personally I have never experienced rudeness from a Khmer, but I have observed it from Chinese and other Asian visitors. In some cases rudeness meets what's perceived as rudeness, but in your in-family scenario that doesn't sound like an explanation. Maybe even more Westerners who live within a Khmer family will take the time to make more appropriate remarks.