The Torah forbids marrying one’s wife’s sister as long as the wife is alive. Even if one divorces his wife, it is forbidden for him to marry his wife’s sister. This applies even to his wife’s half-sister, regardless of whether they share the same father or the same mother. If one’s wife dies, Heaven forbid, then he may marry her sister.
The Shulhan Aruch (Eben Ha’ezer 15), based on the Mishna in Masechet Yebamot (24), discusses the case of a man who hears that his wife passed away, and then marries her sister. Sometime later, he hears that the wife had actually not died at the time when he married her sister, but has since passed on. In such a case, any children born to the second wife while the first wife was alive have the status of Mamzerim, since they were born from a forbidden relationship. Children born after the first wife had died, however, are ordinary Jews and do not have the status of Mamzerim.
This Halacha should remind us of the complexity and severity of the laws regarding Yihus – ascertaining valid lineage before marrying. As in the case described above, marrying hastily before assuring proper Yihus could have disastrous results, to which we may apply the proverb, "Me’uvat Lo Yuchal Litkon" ("A corrupt deed cannot be corrected" – Kohelet 1:15). One must therefore ascertain before marrying that the marriage is permissible.
Summary: It is forbidden to marry one wife’s sister – even after divorcing the wife – unless the wife dies, Heaven forbid.
