I don't see how the mother got a lighter sentence even though she had raped her daughter as well.
Legally speaking, she probably took a plea deal so they could get her to testify against the husband. Or because the prosecutor thought the Jury might not convict her if she gave some sob story about being abused herself. It also depends on how many counts she was charged with. Sentencing is a lot of math and guesswork that many lawyers don't even understand. That's why a lot of jurisdictions have taken sentencing out
of the Jury's hands and placed it in the Judge's, except for in capital cases in states that have the death penalty.
This girl lived a very painful, horrible life and she will likely never
be made whole by the justice system for the things she endured. However, it's important that we also acknowledge the horribleness of the crime and do our best to prevent it in day to day life.
I now work at a family law clinic with about ten clients, all who are victims of abuse in various ways. All of them say pretty much the same thing, "I/we tried to tell someone. He/She/They didn't believe me/us."
Listen to the children.