Which would mean only Jeremy is telling the truth 
or to quote Adams favourite phrase there was a ring of truth. Nothing has to be so black and white . Embelishment of words can be very dangerous in a situation like this.
If Jeremy said yes his sister was familiar with guns from the farm but had not used them on a regular basis - then when the police realised they had made a mistake they had to cover their actions by making out she had used them all - it was their word against his .
so not black and white out and out lies - twisting his words for the "noble cause" - but if that was the case it puts a completely different spin on the situation on the night.
And the family told the police within hours nearly that Sheila could not have used the gun - then why did they not pick up on his blatant lies then?