No, it is completely unethical. Such relationships violate basic moral and family values. In a family, there should be respect and appropriate boundaries between family members of different generations. Incestuous - like relationships are not acceptable in society.
It is not ethical at all. Our society is based on certain moral and ethical codes which clearly state that relationships within a family should be pure and proper. A father - in - law and daughter - in - law are related by marriage in a family structure, and any inappropriate relationship would disrupt the family harmony, cause emotional distress to other family members, and is considered immoral in almost all cultures.
Definitely not. Family relationships are built on trust, respect and proper hierarchy. A relationship between a daughter - in - law and father - in - law should be a platonic one based on family ties. Any form of sexual or improper relationship goes against the norms of a healthy family and society at large.