A stepfather's sons from another woman as non-Mahrams
Your stepfather's sons from other wives are considered Ajanib (men other than a husband or permanently unmarriageable male relatives), not Mahrams, to you. Your children, on the other hand, are considered stepsons and stepdaughters to your stepfather. (Part No. 17; Page No. 372) May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.