If two Jewish sisters marry one's son to one's daughter, are their descendants all mamzerim forever?

Question

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 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 mamzerim?

Answer 1

Marriage between first cousins are allowed in Judaism.

aish.com writes explicitly

It is permitted under Torah law for cousins to marry. In fact, the Sages seem to view marriages between relatives as desirable. The Talmud recommends that a person marry his niece (Yevamot 62b). [...] The assumption is that a person will have an especially close relationship with a close relative, perhaps because of their likely similar personalities and values. The reasoning may be the same for a cousin, but the Talmud doesn't mention it. (I should point out that for a woman to marry her nephew is forbidden from the Torah – Leviticus 18:12-13.)

Some point out that the daughters of Zelophehad married their cousins (Numbers 36:11) so the practice has good precedent. [...]

In terms of the genetic issues, [...] relatives who marry should be even more careful to do genetic screening in advance.

Note that a number of countries and US states forbid first-cousin marriage though.

Mamzerim are the children of an adulterous or incestuous union (e.g., a man and a married woman or of a brother and sister - but NOT of a man to to an unmarried woman). Any decisions on the status of their children need to be considered by a qualified rabbi as there might be ways to disqualify the marriage (e.g., on technical grounds) and avoid the mamzer status.

Answer 2

received an answer from Rabbi David Sperling of yeshiva.co:

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 an unknown (or even doubtful) mamzer is not a mamzer at all. That is, it is not that they are an “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.

Thank you to all who responded!