It happens a LOT, unfortunately. Families like that are so dysfunctional, that most of the players are caught up in their own little world trying to survive the dysfunction.. it would upset too many other balances to actually do something about the abuse. It's terrible to say, but a lot of times members of the family look at another member as the sacrificial lamb kinda thing.. better her than me... that mentality..
My mother and her sister were sexually abused by her grandfather and her mother knew about it, and yet still arranged visits for them. Almost as if she was faciliating.
My ex-MIL (N's mother) was sexually abused by her father. She then allowed her daughter (my ex-SIL, socio/psychopathic, para-schizo etc etc) to visit with him from a very young age, and he of course sexually abused her as well. The SIL never forgave her mother for allowing that contact, KNOWING that he was sexually abusive from her own personal experience.
I don't know the exact dynamics, but it seems to be a common thread. Denial. "it happened to me, but he'd never do it to anyone else..."