Confused whether Rakhi is only limited for brother-sister?

I heard about that only sister ties to brother, and in return brother takes pledge to safeguard his sister from all evils throught her life. Rakshabnadhan is very special festival, that creates a strong bond among brother and sister.

Namaste

Seems, according to mythology, there is no strict rule.

In Bhavishya Puraan, 4/42 Krishn said - "In olden times, in Dev-Asur Sangraam, Daitya got defeated by Devtaa, so they went to Guru Shukraachaarya Jee along with king Bali and told the account of their defeat. Shukraachaarya Jee said - "Hey Daitya Raaj, You should not worry about this, victory and defeat are Kaal's games. Now you extend friendship with Indra, because Indra's wife Shachee has made Indra invincible by tying the "protection thread", that is why you are defeated by him. Now you wait for one year, then you will be all right." So Daanav got satisfied and waited for their turn. This is the wonderful effect of Rakshaa Bandhan, it gives one victory, happiness, son, health, and wealth.

In Mahaabhaarat, an incident is mentioned - When Yudhishthir was doing his Raajsooya Yagya, there was a question who should be worshipped first? Sahadev suggested Krishn's name, most approved it but Shishupaal started abusing Him. As per His promise to Shishupaal's mother He tolerated His 100 abuses and as he abused Him 101st time, He cut his neck from His Chakra. His mortal finger started bleeding by the use of Divine weapon Chakra. Draupadee immediately got up from her throne, tore her valuable dress and tie a bandage on Krishn's finger. Krishn felt obliged and said - "Krishney, I will pay you for this in such a way that the whole world will see and remember." Draupadee was completely ignorant of her future. Later, as everybody knows, Krishn helped her to increase her dress when Dushaasan tried to pull her dress to make her naked.

Source: http://sushmajee.com/vrat-festivals/festivals/rakshaa-bandhan.htm

One day, during the Purnima celebrations, Lakshmiji tied a thread of colored cotton on Bali's hand and prayed with a pure heart for his protection.

Source: http://www.boloji.com/articles/667/rakhi-raksha-bandhan

Pranam

Thank You For Sharing This Wonderful Answer.:)