Question

be动词后面不是应该加形容词吗？那为什么“He is really my brother.”这句话

Answer 1

这里的really用来强调事情的真实性。又如:
Issues
such
as
these
were
not
really
his
concern.
他其实并不关心诸如此类的问题。

Answer 2

be动词后即可加形容词也可加副词
再看看别人怎么说的。