There isn't a law specific to this issue, however, at a certain age boys and girls require privacy. This is usually around 8 yrs old.
The standards are usually set by looking at foster care regulations and what a foster parent is required to provide for children in their care.
I don't believe that there are any laws against siblings sharing a bedroom regardless of age. It sounds like you have a good setup for the room, just use your best judgment.Ask a similar question