The Kitáb-i-Aqdas
133.It is forbidden you to wed your fathers’ wives.   ¶107
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.
Other Related References
Lights of Guidance
Marriage between Relatives, p. 387, no. 1289