Theory time!
This is quite a long read.
The mansion is actually a cover up for a secret cloning laboratory. It was built to look ancient and run down so as to seem inconspicuous. The legend of the witch was also created to keep people away. The mansion is actually a high tech building kept in pristine standards. This explains the outhouse building being so much better looked after. Your cousins are all scientists who have in fact been co-operating with whoever runs the facility and as a result have many clones of themselves. The real reason behind this trip is to visit the "mansion" to do more work/experiments.
However, whilst they slept, the was a breakout at the facility (Oh no!). 3 clones escaped and it just so happened that these clones were of Bob, Fred and Bill. They knew that they were experiments and swore to get revenge on their evil creators. So each clone climbed through the window with a knife to kill their respective cousin. But they were too loud, the original cousins awoke and after a brief (yet quiet) fight, each clone was lying dead on the bed. The cousins then panicked. They never meant to kill anyone. They locked their rooms and hid in the bathroom and locked the door. A nightmare ridden sleep engulfed them all.
Morning came, you awoke. The cousins were too fearful to open their doors to you. As you smashed each window, it was true that the body was dead and that it belonged to one of your cousins. And that you and the cousin were the only humans in the room. Discovering a tragedy, you run and hide under the table...
But wait! There's more!
The secret operation was flawed. A double agent worked at the facility. Someone who was against the evil that happened there. She (yes, a woman. This story had a gender inbalance, ok?) was the one who let the clones out and told them to kill the originals. She watched as the clones were stabbed and the scientists hid. She had failed... Until you came along and smashed the windows in. She went into each guestroom after you had checked, kicked the bathroom door down, killed the cousin and hid the body. When the police arrived, the attention from your story of a witch would prompt a full investigation, leading to the discovery of the secret lab.
Culprit: check
Motive: check
Method: check
