Or same sperm used for artificial insemination…
https://townhall.com/columnists/michaelbrown/2013/02/26/when-you-find-out-you-married-your-sister-n1519841
You knew it had to happen sometime. A husband discovers that he and his wife have the same sperm donor father. (Both of them raised by lesbian parents.) That means he married his half-sister, who is now the mother of their three children. What should they do?