Character Analysis
Bad Grandma
If you think Jeannette's parents are the worst, you clearly didn't read Part III, which deals with the kids living with Dad's mom, Erma.
Erma is a humorless, racist child molester—or, in the words of Mom's infinite delusion, "She's just an old woman who's had a tough life" (3.1.27). Yeah, that's a load of hooey, Mom. Jeannette and her siblings all have horrible lives, and none of them turn out this way.
Erma brings out the best in the kids—who team up against her—and the worst in the parents. Dad defends Erma, even though she molests Brian, maybe because he expects his kids to defend him even when he does terrible things. Which he does on all days ending in "y."
Jeannette tells Erma's story because she wants us to have some context as to why her dad is a terrible person. If you had Erma as a mom, the odds wouldn't be good for you, either.