Who are the mahram for the children born to a man who married his brother's stepdaughter and had children with her?

If a man marries his brother’s stepdaughter, his children from her become mahram to his brother’s children, and marriage between them is therefore impermissible. For example, if Zayd married Zaynab and consummated the marriage, and Zaynab had a daughter named Safiyyah, and then his brother Bakr married Safiyyah and had children with her, Bakr’s children from Safiyyah would be grandchildren to Zaynab (their maternal grandmother) and paternal cousins to Zayd’s children from Zaynab. Zayd’s children from Zaynab would be maternal uncles or aunts to Bakr’s children from Safiyyah, and thus marriage between them would be impermissible. However, if Bakr marries someone other than his brother’s stepdaughter, then it is permissible for the children of each to marry the daughters of the other.