All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His Slave and Messenger.
The answer to the first question is that there is no harm in this woman marrying her son to the daughter of her previous husband from another wife. There is nothing in what was mentioned that restricts such a marriage.
As for the second question, the answer is that it is permissible for this woman to marry her son to the daughter of her brother or to the daughter of her sister. What was done does not legally restrict marriage.
For more benefit, please refer to Fatwa 85868 regarding those who are forbidden from marrying each other, and Fatwa 92178 about how “the pen is lifted from the child until he reaches puberty.”
Allaah Knows best.