Marriage with one's stepmother is here explicitly prohibited. This
prohibition also applies to marrying one's stepfather. Where Baha'u'llah
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-Baha 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. Baha'u'llah states that
it devolves upon the House of Justice to legislate "concerning the
legitimacy or otherwise of marrying one's relatives" (Q and A 50).
'Abdu'l-Baha 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.