Brother just have to be assholes

I know brothers and sisters always argue and fight. That is just a given. Everyone knows that. Everyone goes threw it. Everyone experiences it, even if you dont have siblings, you still see it. But what I dont understand is why sometimes they go BEYOND to be rude, and do it on purpose. Knowing what their doing is wrong.

My brother is 15 years old, and last night he showed my 10 year old sister a video on youtube. It wasnt just any video. One that makes you laugh or anything like that. It just so happened to be titled 'Comet Collision with Earth 2012 Doomsday Planet Nibiru'! Okay he might be really interested in science, good for him. He might really believe this topic, good for him. My sister informed me and my mom that she had a nightmare and was worried about what is going to happen in 2012. Why in the world would you show that video to your sister, knowing she is going to get scared? Knowing she will end up telling SOMEONE? Knowing that someone will blog or facebook about it lol? I just dont get it!