Parashas Acharei Mos · Passuk 18:07
Chabad.org — English translation + Rashi
Torah Temima points to Sanhedrin 54a.
English: “She is your mother” — includes a mother who is not the father’s wife (e.g., a woman the father violated or seduced).
Mother outside marriage
The prohibition of one’s mother applies even when she was never married to the father.