Let us say for a minute, I am best buddies with one of the heirs. If I assume I will benefit by killing my buddy's parents, so my buddy will get richer.
How do I know my buddy, the heir will be okay with me killing his parents?
How do I know the heir will give me money?
How do I know I will not get caught?
If I tell the heir, I did it, (remember we are best buddies), how do I know he will not turn me in?
If I 'cleared' my plans with the heir before hand, he is now culpable of the murders.
The prosecution will consider the heir as part of a conspiracy that committed murder.
Someone associated with the heirs killing the Sherman's, in my opinion is not probable unless they were in a conspiracy with an heir.