To be honest Lookout, this is your route - I can't see any reason why they would ask him if he were the killer but you asked the question. I see you're generalising again!
But I thought the relatives knew Jeremy well enough to approach him on the subject the way they spoke about him even down to the pupils of his eyes ?
Why wouldn't they ask him ? Even if it had been about problems within the famly.Wouldn't you ? Or would you rather accuse someone there and then without knowing them ?