Mikoto/Reito/Obsidian Lord-related, there's one thing that confuses the hell outta me

Since it was revealed that Mikoto's brother is the Obsidian Prince, and that Reito stated that he doesn't have a sister, is Mikoto really the sister of the Obsidian Prince or was it all a misunderstanding and that she's actually the sister of Reito? o_o

I mean, if Mikoto really is the sister of the Obsidian Lord, then wouldn't she have existed more than 300 years or something? Or is it possible that Reito just didn't know that he has a sister during his life before the Carnival started?

This was probably answered before, but my mind is still full of fork @_@

The Minagi family knew about the Carnival and was trained for it by their grandfather from an early age. We only see Mikoto, Reito and their grandfather so it's implied that the siblings' only living relative up to a point was the grandfather.

Reito and Mikoto are brother and sister. They were raised together until Reito had to leave. The fact that a boy was trained is proof they were aware he was the host - This could be a fortunate random coincidence: having two siblings fighting together to restore peace and rule the world. Or tradition in which it always presents itself in two siblings to keep the sister inclined to protect and fight for the brother.
While the issue of when the Obsidian Lord possessed him remains unknown; and going by the Hime logic it was fated and born with him like the Himes were born with their marks and only activated when in contact with Fuuka Academy's proximity; The Obsidian Lord could control and manipulate him.

Reason why, Reito lied when he said he didn't have a sister. He was under another identity and had a different plan from the previous times. So the OL kept Mikoto as a backup plan but had no necessity to have it become known that the two of them were siblings. It could actually hinder his plans.
they are just a young boy and girl, that happen to be siblings and had dormant ancient powers within them that woke up.

He just lied about not knowing her. Several of them lied a lot during the show.