I have avoided reciprocating by calling Muslims brothers or sisters, when one of them refers to me as "brother". I have mentioned that the only way I can refer to them as brother or sister is in the broadest secular human sense, as I would even an atheist, which may have been unkind.
I was recently informed by way of someone who works personally with Muslims that when they refer to Christians as brothers and sisters, they consider us brothers and sisters under Adam and Eve.
In the future I will feel more comfortable to call them brothers and sisters in this sense - stating specifically the sense intended - which is not only true, but is far less insulting to them than putting them in the same group as atheists or agnostics, though according to the Gospel we should be equally concerned for them spiritually.
