Google translates Mein Bruder ist in der ersten Klasse as "My brother is in first grade."
However, Google translates both "My brother is in first grade" and "My brother is in the first class" as Mein Bruder ist in der ersten Klasse.
This suggests that the German sentence is used more often to refer to first grade. However, without context, both should be accepted. Please report this.
Well, it is a much more complex topic than that, as the school systems aren't syncrhonized across countries (not even within the EU, let not speak about the whole world). According to Wikipedia ( https://en.wikipedia.org/wiki/First_grade#United_Kingdom ) American "first grade" is actually equivalent to the British "Year Two".
It would be much more fortunate to focus on the other meaning of the "ersten Klasse" collocation which is the "first class" as in a first class service (restaurant, hotel, train, airplane, etc.)
So how would one say: My brother is in the first class? This is a sentence that is meaningful in English. Examples: My brother was in the first class (to graduate from our high school). My brother was in the first class (of the day). Also, in many American schools, they divide (or at least did divide when I went to school) each grade into Classes by ability / curriculum. My brother was in the first class (when he was in 7th grade). There are many such examples I could provide.
Because of this, it seems to me that "My brother is in the first class." should be accepted as a translation, unless there is another way to say that.