Wow, that may be fine in rural areas but in urban areas where rents are high people live together out of financial necessity.
Brothers in Christ should suffice.
Federal law can trump this, all they have to do is state that they are living there as a FAMILY.. doesn't matter what their blood relationship is, could be NONE... Federal Housing law prohibits the local government from doing anything the way its written.
If the people living their claim to be residing together as a family, even if they have no blood relation at all, they are protected under Federal Housing Codes... this town is about to get a HUGE JUDGEMENT AGAINST IT if it tries to enforce this.