The timeframe is hard to wrap my your head around. Coming up with a plan and disposal of a body is pretty hard to carry out and not easy to get away with on the spur of the moment. And if they did get rid of the body when the FM sister was being picked up at the airport that night, that is pretty brazen whilst police are still searching/ have you under suspicion.
Yes, maybe a seasoned criminal would be able to carry that out, but some middle aged women all on her own!!! And wouldn't the police officers and first responders pick up on body language ? Aren't they trained in that sort of thing to look out for clues?