I'd have to know what the cops know about the son to lean towards him being involved. It just seems too risky for a supposedly intelligent person to contemplate, but who knows. I lean towards someone who heard a domestic worker say something about the woman and her daughter. They need to interview all the people who have been in the house over the past couple years. I heard they are doing that AND taking DNA. But I don't think it was an actual worker, I think they told someone. Sometimes hired help live in "communities" and word travels.
I think you’re right. it’s likely that it was someone who worked for her or knew someone who did. That’s a big place for an 84 year old in precarious health to maintain, and it looks very well cared for. She probably had several people working for her who would have knowledge of her home and routine.