I know SY has said there is unidentified DNA in the house.Police have very good reason to suspect another person in that house that night and if DB was concious she knows who was there. That has got to be one of the major reasons the boys are being interviewed and swabbed. That would explain the need to interview the parents separately. I think the reason DB was not forthcoming about the wine initially is that she's not supposed to be drinking and did not want to tell her husband--people who drink are not usually honest with those who think they have a drinking problem.
One other thing: dead bodies do not just disappear. There is evidence of death all around when there is a death scene. MOO
Except JI has said he is always home at night but this one particular night when he did the side job and DB said to MK that she drinks 2 or 3 times a week (IIRC), so JI would know she drinks.