^ETA: beat me to it!
He told her they'd break up, he specifically avoided telling her he'd be murdered. IIRC when she asked about John, Al said "don't tell her..." which is what lead Sam to divert her with a rendition of 'Imagine'.
It's one of my favourite scenes too. It starts of so light-hearted and charming and then the camera just fixes on the girl's face as it subtly changes from humouring her brother to the mortal dread that he's telling the truth about their older brother dying. Quite powerful and very well acted by such a young performer.
I can't help but wonder what her reaction was two years later in '71 when that actually came on the radio...