What you said: " They could be related. B ut they could be siblings too."

What that means: "They could be connected by blood, but in addition to this/on top of that, they could be brother and sister."

What (I think) you meant to say: "They could be related. They could even be siblings."



What do you mean by " If not siblings, then they would be mates"?

What you are saying here is that the consequence of them not having one or both parents in common, is them being romantic partners.

This makes no sense. Can you see why I am confused?