What is the law on the age brother and sisters sharing a bedroom

In what state?

Normally there is no "law" there are rules and guidelines that family and children services may use if they are investigating a complaint of some sort.

There may be housing rules of local housing authorities

Why the concern, what is happening to make you worry.

As noted, if there are any laws about this (there usually aren't), they will be on the local level, not even statewide.

Generally, the local family services agency MAY have guidelines about this, but they would only be used in case of an investigation.