Bruder is singular, but when you add umulauts to the u, it becomes plural
The plural is Brüder (with umlaut). Some words have the same plural (eg Spieler) These are usually masculine or neuter so that you can tell the difference from the article (eg mein Spieler/meine Speiler). But in this sentence the big give away is "er ist" which will always be singular.
Why not "Er ist meinen Bruder"?? Shouldn't we use the accusative case for a masculine noun
Ooops! I wrote Er isst mein Bruder and it was marked as correct but surely that means 'He eats my brother' and surely that is not the intended meaning of the phrase spoken?