Sorry if anyone thinks I post here too much without giving others enough chances to post, but it's just too fun :p
Excellent. Here comes your doom then: One of the cousins exited his room through his window, and proceeded to knock on one of the cousins window, who opened and he entered. Then, he killed him, and using rubber band he was able to make it so that when he closed the window (from the outside) it would lock itself. He then exited back through the window and made the window lock itself. He then did the same with the other cousin, and then returned to his own room, locking the window. However, while he was inside one of the other rooms, another person had sneaked into his room through the window. This person now killed this cousin, and using a similar rubber band trick, he then exited the guest house through the window, letting it close behind him.
Try to dodge this blue truth!