Ages 6 to 8. The brother is younger than sister.

What age should brother and sister not be in the same room.

As far as I know, only public housing (gov't) has rules about this. Otherwise, use your judgment as they grow. Usually they start building little barriers between their beds to create their own space anyway.

It is also a matter of local law.

No laws

They can always share a room if needed, many people don't have large apartments or houses.
So often use bunk beds and so on.

But the idea time would be about school age. It will be more of a social norm based on your culture background.

In general there are no laws about sharing a room. Is there more to this story, is there a ex spouse causing trouble, is there a government agency invovled?