Two siblings that make love to each other.

Briefly, "the realization that each random passerby is living a life as vivid and complex as your own." Originally from the Dictionary of Obscure Sorrows, which has a lot more beautiful neologism definitions like this you might enjoy.

A friend whether a boy or girl that is so close to you, it like you're siblings with different parents.

Somebody who is not your sibling, but you would call them that.