Shalom Rabbi, I’ve heard that the children of two sisters are like siblings spiritually. If a woman gives her son to marry her sister’s daughter this is forbidden, correct? Does this make the union incest and the children of such a union mamzerim? Even if this was done not because of some icky sex crime like a brother raping his sister, chas v’shalom, but because it was very common to marry cousins in the shtetls of Europe? But the first cousins who married--- their mothers were sisters. 1. Halachically, are the children of such unions mamzerim? 2. If a mamzer is unaware of his or her status and marries a regular Jew and has children with this Jew, are their children a mamzerim? Thank you so much for your time.

Shalom, Thank you for your question. First cousins are allowed to marry according to Jewish law. The children of two sisters (i.e. first cousins) are permitted to marry, and therefore there is no question at all of any mamzer status applying. [However one should be aware that there are genetic issues involved in such a marriage – and one would be advised to seek genetic testing etc before such a union.] I am unaware of any source that equates cousins of siblings. In regards to your second question – the law is that a unknown (or even doubtful) mamzer is not a mamzer at all. That is, it is not that they are a “unknown mamzer”, but one of the laws of a mamzer is that until the mamzer status is clarified beyond doubt, there is no mamzer at all. Blessings.