Well yes irmãos can mean brothers, but it can also mean siblings which is correct
"My mother and he are siblings" is marked by Duo as incorrect, while "My mother and him are siblings" is marked correct. In fact, the former is grammatically correct and the latter is incorrect.
there is something wrong with this sentence "He and my mother are brothers" which is correct to Duo
The English word "brother" includes males only. If the group includes different genders, you must say either "brother and sister" or the inclusive term "siblings".
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 :)
In Portuguese, "irmãos" means "brothers" and "brothers and sisters" too.
I think because "he" is mentioned first before "my mother" and it doesn't accept it if the translation is not in the correct order, at least from my experience
In this case, you can't use "him," since it is a pronoun in the subject position, not an object position. e.g. "I gave him the plate of food." and not "I gave HE the plate of food."
Is the sentence "him and my mom are siblings" grammatically wrong or why doesn't Duo accept it?