In the state of Wisconsin, is there any law prohibiting opposite-sex siblings from sharing a bedroom after a certain age?

As long as a child lives in the parents home, I do not think there are any laws that would force a certain bedroom arrangement, regardless of the age of the children.

There are no laws that effect a brother and sister from sharing a room BUT>>> with that said, if you are going though a divorce, the judge in that court may look down on it, if you are looking to adopt, the home inspector will not approve it, if you are being investigated by Children and family services they will most likely not approve.

And to be honest at a certain point, they will both some more privacy.