If you put SH's involvement aside for a moment and think about who else might/might not be involved then *IF* the visit to the fair didn't happen then it's very relevant when the story emerged, both to the rest of the family and to the police wouldn't you say?
DS must have a pretty solid alibi as his whereabouts have never been discussed. I wonder if he does live there at all though as nothing's been mentioned about him coming home on Thursday evening and he's not even mentioned for Friday so I guess he doesn't eat his meals in the house either.