"Your brother is writing a letter."
Translation:Twój brat pisze list.
Accusative is a tricky thing for masculine singular nouns. When the noun is animate, the Accusative form has its form identical to Genitive. (with the exceptions of tata, mężczyzna and such).
When the noun is inanimate, the Accusative form is the same as the Nominative.
And then a lot of foods/beverages are grammatically animate because of absolutely no reason at all.