Are the stepmother's sisters forbidden to the father's sons from another wife?

The stepmother's sister is not a forbidden (mahram) relative to the son, and it is permissible for him to marry her, as Allah Almighty says: (And lawful for you are [all others] beyond these). The Permanent Committee for Scholarly Research and Ifta and Sheikh Ibn Baz — may Allah have mercy on him — have affirmed the permissibility of this, stating that she is not forbidden to him; rather, she is a non-mahram (ajnabiyyah) to him.