I think they couldn't have left the properties of such close relatives unsearched, not if they had family members on record saying that they had been digging on their property. If they happened to have some other evidence that points to someone and they decided to make an arrest without a body it it might create reasonable doubt for the defense if they could point out relatives without an alibi and ready made holes they could have easily buried the body in.
I was aggravated that this more thorough search was done now and not the day after Haleigh went missing but then again, *if* Haleigh was buried somewhere we don't know for certain when. They did say they had some reason to suspect she might be alive, a couple of months into the search. I don't know, maybe it was just because they hadn't found her dead but if someone abducted her for some other purpose than to kill immediately they might have kept her alive for some time and then got rid of her when it became too dangerous to keep her.