I guessed I must've missed it when we learned that HG's sister had packed a bag to stay over night but instead went home after work. So it turns out that three different family members were supposed to stay in three separate homes there that night but all changed their plans. Otherwise there could've been eleven victims instead of eight?
Do you really think HG's sister would have stayed home knowing her own sister was about to be killed? And not try to find a ruse to get her sister out of there? That would be pretty cold, I can't imagine she would do that.
Maybe it's kind of like what happened after the Manson murders when dozens of people claim to have been invited to visit Sharon Tate that night, but changed their plans. It's some kind of psychological reaction to a horrific tragedy that happens to someone you know well.