No offense taken. XD It makes sense for people to put their own thoughts and beliefs in their characters because it is difficult to have a completely fresh character with all new bias that you've never experienced before from anyone else or yourself. I try to be different with some of my characters (self-insert, not included XD)
It is completely fine for someone to think they are siblings, although someone can see the same cloaked figures and think they are dating or are really great friends. It's how you perceive it and, generally, people who are of an open mind tend to find it easier to make characters that have a different perspective than they do, but that doesn't make it any less challenging.
Even though meta-gaming and meta-roleplaying is a thing, where you just know what they are because the roleplayer read their history and, as such, their OC knows the other character's history, even without hearing or knowing them from before or from anyone else, you can have your OC know if they are dating, friends, or siblings because of their perspective. That's fine. And besides, most of us all have some kind of belief or perspective that our OCs share because, in some aspects, they are extensions of ourselves, whether we make them completely like us or not.