Are step-siblings Mahrams to one another

Siblings who share the same mother(Uterine siblings), for e.g. one’s mother married two husbands and she had children from both husbands, or siblings who share the same father (Consanguine siblings), for e.g. one’s father married two wives and he had children from both wives, fall under ones unmarriageable kin (Mahrams). For these categories observing the laws of Hijaab is not necessary.

On the other hand, siblings who do not share the same parent but are from previous marriages, for e.g. one’s mother/father married a second husband/wife who had sons/daughters from a previous marriage fall under ones marriageable kin (non-Mahrams). For this category observing the laws of Hijaab is necessary. (Badaai-us-Sanaai 2/257)

And Allah Ta’ala Knows Best