What kind of relationship do you have with your opposite sex sibling?

[CCP]

So i really liked this girl and talked to her in school a little. i was about to send her a friend request on facebook when i saw her profile pic was of her and another guy hugging and cheek to cheek. I looked at a few other pics and there were others like that. So i askeed her friend did she have a BF and she said no. I came to find out that it was her brother. at first i thought it was weird.
But then another friend of mine came over and his sister came too and they were joking about sex (not with each other) and i thought that was really weird to talk like that to your siblings. then they posted pictures on facebook of them at a party drinking out of the ame cup with straws and taking pics.
Usually me and my siblings fight so idk whether stuff like that is normal or not