Fair housing applies to apartment complexes also.
Back in the mid eighties an apartment owner could set aside one building for families and one for folks without kids... but that changed a few years later.
But even in the no kid building there was leeway for kids visiting non custodial parents, and for military on leave.
IIRC the guest rule was for exactly that, guests, not family.
The landlord never charged extra when the family member visited.
And this was in California; personal experience.
“Fair housing applies to apartment complexes also.”
It does not apply to roommates and shared housing such as this.