What is the ruling on a legal marriage between a girl and her brother, without the knowledge of her father due to personal disagreements, given their subsequent mutual consent, and is the brother or sister sinful if the brother becomes his sister's guardian during her father's lifetime?

It is not permissible for a brother to marry off his sister in the presence of their father, and personal disagreements do not justify this. This marriage necessitates annulment and renewal of the contract if the spouses wish to continue. The sinfulness of this act depends on the doer's knowledge of its prohibition; whoever knows and has no interpretative justification is sinful, while whoever is ignorant or has an interpretative justification incurs no sin.