That's a careless translation: a "brother-in-law" of someone who has never married and has no married siblings. Ouch!
Please use a dictionary. Please.
Just because the term used can mean "brother-in-law" in English does not mean that's how it should be translated in every given case.
From the context it should be blatently obvious that the term used should be translated as either "adoptive brother" or "foster brother." (Since the parents of a previously only child — namely, Bridgette — took another child in.) The translation is otherwise very good, which makes such carelessness all the more jarring and obvious.