Well, I've been told by several different Russian text books that just leaving out the adjective and letting it be clear from context whether the person was a sibling or a cousin was a common thing Russian speakers do. So I assummed it was standard usage.
Also, a Russian podcaster I listen to (iirc from near St. Petersburg), who does podcasts where he doesn't do grammar or such a lot but just casually talks relatively clearly and slowly with a lot of rephrasing to explain words, talked about a conversation he had with his mother about a "брат" he didn't know, the kid of some uncle or something, and then he did a brief aside explaining the usage. So that guy did it in fairly normal speech.
My RL teacher in my course (originally from St. Petersburg) just told us to use cousin (which you do in German as well as the older native German words are now really uncommon), and about the in-law situation he basically just mentioned it briefly, said sympathetically that it's a mess and that he also always has to think a moment which is which because he doesn't have any in-laws himself so it never comes up for him.
no subject
Also, a Russian podcaster I listen to (iirc from near St. Petersburg), who does podcasts where he doesn't do grammar or such a lot but just casually talks relatively clearly and slowly with a lot of rephrasing to explain words, talked about a conversation he had with his mother about a "брат" he didn't know, the kid of some uncle or something, and then he did a brief aside explaining the usage. So that guy did it in fairly normal speech.
My RL teacher in my course (originally from St. Petersburg) just told us to use cousin (which you do in German as well as the older native German words are now really uncommon), and about the in-law situation he basically just mentioned it briefly, said sympathetically that it's a mess and that he also always has to think a moment which is which because he doesn't have any in-laws himself so it never comes up for him.