The translator of The Count of Monte Christo threw me a curve. A girl whose mother is dead, calls her father's second wife her "mother-in-law". I looked up in French English dictionary, could not find French for "step-mother".
So I looked uo defintion of mother-in-law, and found French uses same word for both "mother-in-law" and "step-mother".
belle-mère
Nom féminin

(a) mother-in-law; stepmother

So the translator goofed and chose the wrong definition!