Answered by Ustadha Raidah Shah Idil
Question: My non-Muslim father cohabits with a woman. Would she become my mahram if they had intercourse, even without being married?
Answer: Assalamualaykum wa rahmatullahi wa barakatuh,
I pray this finds you well. May Allah reward you for wanting to do that which pleases Allah.
No, his partner is not your mahram. If he marries her, then she becomes your step-mother, and will become your mahram.
Please refer to the following link:
Checked & Approved by Shaykh Faraz Rabbani