Karr sets up the entire point of this memoir (and, apparently, the next two memoirs) via the introduction by pointing out that all families are dysfunctional, in some way, mainly because we're all human and we all have secrets and we all have things we fear. It's the degree of dysfunction that makes her particular life so engaging to read about.
Now, there are parts of her life story that are abysmally awful, that no one would wish upon anyone. They are, likely and sadly, common tales, especially from young girls - tales of sexual abuse. Karr treats these parts of her story differently from stories about her mom and dad and sister. She just... describes them. And lets the reader decide how he/she feels about their impact on Karr's life.
Everything else is treated so differently - poetically, in fact. It's not surprising that Karr is attempting to wring meaning out of why her mom would throw all their dresses on a big bonfire. Or what it meant when her dad told slightly fallacious stories to his friends at the Liars' Club. Or how her sister's personality helped shape who Karr is as a person, inside this family.
So the difference in treatment comes down to this - you can't wring meaning out of sexual abuse. It happened, and if you're lucky, you can move on. But the family memories - of what you did together as a family - those make you who you are and help you grow and understand yourself throughout your life (and their lives).