Because an Portuguese, people can (and frequently do) throw articles in the middle of a sentence and it's still grammatically correct.
There is usually an article before possessives, especially in Portugal and the other Portuguese-speaking countries outside of Brazil.
the alternative correct solution "his brother's drink water" sounds odd. i'm no native english speaker though. whats wrong with "his brother drinks water"?
His brother, her brother, your brother, their brother, even technically, its brother are all covered under the "seu" possessive.
But not "his brother's" (unless you are talking about perhaps his brother's wife, children, car... but not what he drinks, unless it is perhaps his brother's whiskey being shared by all :D). That however, would force a different sentence structure.
You could use o teu irmão and, o irmão dele/dela(s) to eliminate some of the ambiguity of the 3rd Person declension of "seu" here.
However, seu irmão is acceptable for all those listed in my first sentence. So DL should accept for all those translations.
because your is very different his his - dele| deles your - seu| seus
"his brother drinks water"- O irmão dele bebe água "your brother drinks water" - O seu irmão bebe água
Seu in this case is used in the second person, his/her is third person. This sentence in the third person would be "O irmão dele bebe água."
Seu is always the 3rd Person declension of the possessive so it covers his/her, yours (você vocês), theirs, and its.
While "dele/dela(s)" clarifies the ambiguity of "seu" it is still 3rd Person declension.
The confusion comes in that "You" in English is 2nd Person (singular). It is in Portuguese as well, but only in the "Tu" form (same with French and Spanish and most Latin languages outside of Brazil – which is eliminating 2nd Person Singular altogether – but even still in many places in Brazil).
"Você" is the formal version of "Tu" and was used as if talking to nobility, 'Would his majesty like another serving of king cake on his plate?' So because it is an indirect way of addressing someone, it forces the 3rd Person conjugations and declensions of the rest of the sentence.
- eu bebo (1st Person)
- tu bebes (2nd Person)
- ele/ela/você bebe (3rd Person)
- nós bebemos (2nd Person Plural)
- eles/elas/vocês bebem (3rd Person Plural)