The daf has been discussing who is allowed to marry who. One category it mentions is a “foundling.” A foundling is a found-child who does not know who their mother or father is. There is a debate about who they can marry . . . and it get’s pretty strange.
And for what reason did the Sages say that a foundling is unfit to marry a priest, levite or Israelite ? Lest he marry his sister from his father. The Gemara asks: If that is so, it should not be permitted for a foundling to marry even a female foundling, lest he marry his sister from either his father or his mother. The Gemara rejects this: Are they continually throwing away all these children? Is it likely that the same parents abandoned both a son and a daughter? If you accept that suggestion, it should not be permitted for him to marry the daughter of a foundling, lest he marry his sister, as perhaps the father of the one he wishes to marry is his father as well. Rather, it must be that it is not common for a foundling to happen to marry his sister, and therefore he may marry the daughter of a foundling. So too, it is not common for him to happen to marry his sister, and the Sages would not make a decree to prevent this from occurring.
Wow! Okay, so the rabbi who is forbidding the foundling from marrying into the community is worried they will accidentally marry their own sibling. This is shut down by the realization that, if that’s the worry, they shouldn’t be able to marry anyone (that is not a Jewish thing, abstaining) and it’s super unlikely anyway.
So, of course I have to report
This story of brother and sister split up “parent trap” style only to find their siblings when their parents meet on their wedding day!
This story of a husband and wife discovering they’re siblings when they’re daughter was 6 years old (they were both searching for their moms who abandoned them as children – only to find out it was the same woman).
This one where a woman found out her boyfriend of 6 years is her brother.
This story where the married couple turns out to be TWINS!
And these two young women who were dating for two years before finding out they are siblings … and didn’t break up.
So, is it common? No. But we certainly don’t need to worry about foundlings anymore as we have genetics to tell us who our siblings are. . . but even still, mistakes happen.