Jeremy got a call from WHF and he recalls the words as "Please come over. Your sister has gone crazy and has got the gun."
Not got "a gun" but got "the gun". So IMO either Jeremy thought that was his father talking or he knew who was talking to him.
In that short conversation, the caller did not say who he was. How often do we all not recognise who is speaking to us especially if we've been woken from a sleep?
Someone else has the gun, not Jeremy, so he doesn't fall for the trap, for if the murders were in some way planned the best thing for the third person would be to eliminate Jeremy as well for it is only him who knows.
But Jeremy doesn't fall for that and calls the police. That move saved his life but it doesn't put him out of danger.
In Andrew Hunter's Draft book on the murders at Whitehouse Farm there is substantial detail showing that there were odd bullets that were used that night, they were heavier and didn't break up when fired as did the hollow point bullets that were at WHF.
This makes one think that same gun may have been used, but different magazines were inserted once the first 10 bullets were used. Some of the shells showed they were extracted from a magazine twice. That is something Sheila didn't do for certain, and nearly in every occasion one bullet of this type was used on each deceased person. As I read about this I had the feeling the third person went through the house and used one shot in each person (except Sheila) to make sure they were dead, doubly dead.
There was no way Sheila would have thought like that when she was just suicidaL.