How Raki festival came?

We do not know origin of this festival. Rakhi worn by brother is token of his promise to protect his sister. Historically, A Rajput woman tied Rakhi on emperor Humayun.

http://festivals.iloveindia.com/rakhi/significance-of-rakhi.html

This festival reveals weakness or frailty of women who depend on men. This also shows strength of brothers who assume responsibility for protecting sisters. Ironically, even a strong police woman will tie Rakhi to seek protection even from her weak or lame brother.

Rakhi is the only festival meant for brother sister relationship

I agree with Gulshaji that in history it was first tied by Rajuput women and the trend continue without any caste or religious differences. The reason for the festival can be seen in two ways 1) brother protecting the sister 2) sister asking well wishes and protection from God. Hence it is a bond of love and care.

We have seen in history that jodha tied rakhi in the hand of her brother Sujamol. It is a bond of brother and sister with love and care.

wow , thanks for the explanation and the clearing my doubt

I dont know about the existence but it is to show love and care in brother sister relationship.

What do you mean by how rakhi come it does not nicely understood please give details.