What is the Age when a Brother and Sister Can't Share a Bedroom

At what age is it that a brother and sister have to stop sharing bedroom?

There is no such age.

If this is an issue in a custody case, the parents can try to come to an agreement or may raise the issue in court. However, one parent's being unable to afford a home with an additional bedroom is not going to be a basis for a change of custody or denial of visitation, nor will a court disallow a family trip merely because siblings may share sleeping quarters.