"Notre beau-frère termine son livre."
Translation:Our brother-in-law is finishing his book.
Yes but according to a French native:
we tend not to use "beau-frère" for "stepbrother", to avoid confusion with "brother-in-law". We use a circumlocution such as "le fils de son beau-père ou de sa belle-mère)" (beau-père and belle-mère being themselves confusing, as they mean step-mother/father, as well as father/mother-in-law)