Festro's nephews appeared in the episode What It Do Nephew?. Their names are Huey, Louie, and Festro Jr. Huey is green, Louie is pink, and Festro Jr. is yellow. Festro believed they were going to be "crude dudes with attitude" considering they were the sons of his brother who is supposedly to be "super tough", but they ended up being scrawny wimps. This gives us more insight into Festro's family and disgustoid culture, as Festro's brother forced his sons to be wimps because he is cothe only "#1" in the household. This could only imply Festro's family is highly territorial and thrives on a system where there is one alpha male and the rest must follow on as "#2", or weakling.
Festro's nephews seem to be afraid of anything that's not sleeping or eating. Whatever the bros would do to try and toughen them up, the nephews would either get scared or complain. It is heavily implied that the nephews were afraid of their father because whatever Festro told them to do, they did it immediately no questions asked. Festro's brother probably would threaten them. This is another example of the Alpha male society.