Final answer for why
"Hindu, Muslim, Sikh, Esai
Aapas me hai bhai-bhai"
And why not "behn-behn"?
Though I am not aware, whether “community” is consider as male or female, as per Hindi Grammar (as posted by Abhishek), but in spiritual terms, the community and its members, irrespective of gender (soul the children of God) were considered as male.
In India as well as in most other countries, property is inherited from father to son. Son is made responsible to see that his parents, wife and children live in a healthy, wealthy and peaceful environment. He knows that God is the ultimate owner of all these possessions. With the anticipation of these properties as inheritance from God, he venerates Him as God Father / Supreme Parents.
As a physical (body) form, we have individual parents, but as souls we are the children of God. Though soul can be termed as either gender, but keeping the unlimited property as inheritance from God Father in mind, soul is termed as male and accordingly we are all brothers.
Hence it is “Bhai-Bhai”