I saw this in an episode of "Autopsy". Karla confessed that she drugged her sister til she passed out and let Paul rape her vaginally and anally and then Paul asked Karla to perform oral sex on her passed out sister, and she did.
During the whole thing, her sister started vomiting (she was drunk as well) and choked to death on her on vomit.
The apparently were guilty of a whole list of horrific crimes.
http://www.crimelibrary.com/serial_killers/notorious/bernardo/homolka_jump_page.html
PAUL BERNARDO & KARLA HOMOLKA - In-depth feature story on the notorious Canadian serial killer team known as the "Ken and Barbie of Murder and Mayhem." After a highly controversial "Deal with the Devil," the Canadian government was forced to set Karla free. The Canadian government and people are now on a "Karla Watch" to see if she will try to make a quiet life for herself or kill again. A new film exploits her role in the torture and murder of school girls.