Well then, from the beginning:
Before the story in the first post, Bill, Bob and Fred agreed to scare the detective by faking their deaths. So when the time for the travel came, they all had a fake body organized and stored in the trunk of the car.
When he was sure that everyone was asleep, Bill (culprit) went outside, snuck into the mansion and killed Bill (owner) in his bedroom. Due to natural heritage, Bill's (dead owner) son is now the new owner, sleeping peacfully in his own bedroom.
Early in the morning (5 o'clock seems apropriate), the three cousins all went out of the windows to fetch their fake bodies, placed them in the bed, poured some blood around, hid in their bathrooms and waited for the detective to wake up.
After the detective made his drama, Bill went to kill Bob and Fred.
At around the same time, Bill's (dead owner) son found the body of Bill (dead owner), saw no one in the room and immediatly locked himself in to hide from a possible assassin.
I think I covered it with that.