Aunt P may have shared childhood experiences of abuse- I can see two sisters coming through something like that with a strong bond and even stronger sense of protectiveness for each other. Almost as if she couldn't protect her sister from what was happening then, but she can help protect her now. With PR having also battled stage 4 cancer for so many years, PP may have done this because she felt her sister just couldn't survive what would have happened if she'd been prosecuted. PP was supposedly close to JBR- she may have been close to BR as well, and not wanted to see him watch his mother be prosecuted for his sister's death, knowing that one day he'll suffer his mother's death.
It is interesting to note that the third sister does not feature much in this case. I rarely see her mentioned, rarely see any comments from her; possibly she is a more private person than her two beauty queen siblings or maybe she is just reluctant to play a part in the charade. She may know the truth and feels she'd rather stay silent than lie publicly.
