"Mon beau-père est le mari de ma mère."
Translation:My stepfather is my mother's husband.
Indeed. Without context, it's a correct translation. One may argue that it's circumstantially unlikely, but we have such sentences as "Steel is not a metal", "The duck reads the newspaper", "France is a democracy", "The house was destroyed by the angry badger", and "My parents love me".
So these things happen. Ours is not to question how credible or not the statements are, only to render them into another language. Maybe the speaker's father-in-law is indeed their mother's husband.
The exercise is not intended to portray a kinky in-bred family. It is intended to be as straightforward as possible, therefore we need to examine the entire sentence to understand what it means before we fix on "stepfather" or "father-in-law". When it says that (whoever) is my mother's husband, then that is not your father-in-law because unless there is something very weird going on, that's not what a father-in-law is. That's your stepfather.