It is forbidden you to wed your fathers' wives.Marriage with one’s stepmother is here explicitly prohibited. This prohibition also applies to marrying one’s stepfather. Where Bahá’u’lláh has expressed a law
between a man and a woman it applies
mutatis mutandis as
between a woman and a man unless the context should make this impossible.
‘Abdu’l-Bahá and Shoghi Effendi confirmed that, while stepmothers are the only category of
relatives mentioned in the text, this does not mean that all other unions within a family are permissible. Bahá’u’lláh states that it devolves upon the House of
Justice to legislate "concerning the legitimacy or otherwise of marrying one’s
relatives" (Q&A
50). ‘Abdu’l-Bahá has written that the more distant the blood-relationship
between the couple the better, since such
marriages provide the basis for the physical well-being of humanity and are conducive to fellowship among mankind.