My initial thought was revenge killing by someone close to the family who knew the layout of the house, but that was before I knew that the house did not have a camera surveillance system, and that the pool camera was disconnected.
Garland was a life long criminal and mentally ill. He was a 5'7" and he managed to put 6' Alvin on the bed of his truck. The Liknes couple was leaving town within days of their murders, similar to this case. A significant difference is that Garland made no attempts to sort it out with lawyers, but Barry's cousins have had this in the courts for years. If the Sherman's were murdered, then the motive could be the same as with the Liknes couple.