Did Oslo murderer have accomplices?

Oslo police are looking at whether the man accused of murdering his three sisters had help:

Investigators are now trying to determine if it was possible for one person to kill all three sisters without any of them managing to escape from the large house - the sisters were found at different spots in the house.

I'd be taking a close look at the guy's two brothers...it appears they were at home at the time of the murders. And as we all know, honor killings are a family affair.