"My step-brother is kind."
Translation:Mon demi-frère est gentil.
My problem is that Duo is highly inconsistent with what the terms beau-frère & demi-frère mean, especially where, as here, there is no context whatsoever.
Without context, beau/belle- can indicate either an in-law or a step-relative. Without context, demi- can indicate either a step-relative or a half-sibling. Since there's no other way of telling, Duo needs to get it's act together and decide what each means when context is lacking.