Cathy stabbed her mother!

Obviously Catherine killed her mother. Elizabeth was stabbed 37 times, that's a crime of passion, a sign the murder hated the victim. I believe Homicide Detectives refer to it as "Overkill." Robert was an amateur body builder, he could of easily walked up to an elderly Korean woman and slit her throat without much of a struggle.

My guess is, Cathy entertained the notion of killing her mother when she found out she wasn't getting any inheritance, she gave it thought but didn't follow through. One day at the dry cleaners she and her mother get in an argument, it gets heated, then Cathy snaps. Maybe from all the rage she had at her dad, she projected it onto her mother, or maybe she hated her mother for not standing up to the father. Anyhow, Cathy then stabs her mother to death.

Whether Robert was there or not is anyones guess. At the very least he was complicit. Andrew is the last victim, in the whole mess. Manipulated by his sister into killing her fiancee.