The University of Texas, Austin rule states, in part:
No University employee may approve, recommend, or otherwise take action with regard to the appointment, reappointment, promotion, salary or supervision of a close relative as defined by this policy.
which is not, I think, uncommon.
So: No hiring your spouse or supervising your spouse.
There is also some weasel language that could potentially undercut the policy in practice. If you become married or a spouse transfers under your putative supervision, there has to be notification but it is allowed. The the management and oversight of this nepotistic employee goes to the PI’s boss.
This is likely how a thin veneer of red tape covers the case of a spouse working in the lab of an appointed Professorial rank person.
So nepotism is officially bad, but University policy has enough wiggle room to permit a de facto case of hiring and supervision of one's spouse.
In a lab the idea of getting meaningful supervisory oversight from the PI’s supervisor is a joke. It in no way can mitigate preferential treatment and in fact justifies it. The PI can set work hours, discipline for poor performance and even fire everyone *except* the spouse.