A man's stepson is not a Mahram to the man's other wives
One of my wives has an orphan son who has been living under my guardianship for more than fifteen years as I married his mother when he was under three years of age. On the other hand, I have other wives, sisters, and a daughter; should they put on their Hijab (veil) in the presence of the concerned boy?
It may be worth mentioning that I consider him as one of my sons and people regard him as such. Please provide me with your beneficial answer. May Allah safeguard you.
The concerned boy, i.e. your stepson, is not a Mahram (spouse or permanently unmarriageable relative) for your other wives, sisters, or daughters (who are not born by the boy's mother). It is thus Wajib (obligatory) on all the women mentioned above to put on their Hijab in his presence. Moreover, it is impermissible for such a boy to be in Khulwah (being alone with a member of the opposite sex) with any of them. May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.