Yes, it is strange ("he and my mother are brothers") , but I can think of ONE scenario it could be used: If my mother is a transsexual woman (born a man) and she and my father adopted me - or she moved in with my widowed/divorced father when I was a young child - and I still cannot accept that she is a woman (even though I call her mother) so I say that she and her brother are brothers (because I'm disrespectful and transphobic). Then this sentence would make sense.
From what I understand, a lot of people go to Brazil to go through their sexual reassignment procedures (hormone replacement and/or operations) because their laws are very accepting of transgendered people. So sentences like that might come to pass in Brazil after all. Please don't take this as a transphobic comment. I'm transgendered myself :)