The thing is that it would be very strange to say " He drinks milk with THE brother", since a person is only a brother in relation to another person: his brother, my brother, your brother. People who are members of religious "brethern" would be called The Brother. The Marist Brother, The Christian Brother etc. As Zchbaniel25 said at the top; it is a very improbable sentence but not actually wrong.
When brother's friend used to come over, his grandma would drop him off, and his mom would pick him up later. His grandma would always say, "The mother will get him later." It always sounded to me like his mom must have been his grandma's son's wife, toward whom she had a deep hatred or estrangement. Or they hadn't met? Maybe it's different in German, but in English, I would never use "the" for a person I knew unless I was trying to say they aren't human.