Translation:My stepfather is my mother's second husband.
What's the difference between deuxième and second/e?
Correct me if I'm wrong, but even though "beau-père" translates as both "stepfather" and "father in law", the two concepts are different in English. The second husband of one's mother would indeed be their stepfather rather than their father in law. By coincidence, I guess one's father in law could become their stepfather, but I assume that it's fairly uncommon.