[NSFW] Can someone explain to me the following piece of information found on the web?

It’s related to the “Dollanganger” series by V. C. Andrews. It’s mentioned on TV Tropes. Here it is:

In the fifth book and prequel you find out Cathy’s parents were not half-uncle and niece…they were actually half-brother and sister AS WELL as half-uncle and niece due to Cathy’s grandfather Malcolm raping his stepmother Alicia.

Someone must explain it to me.