Raksha bandhan is that a sister tyes it to the brother .
becuse in olden days the poros wife was a pretty girl when alexander attaked the poros then is wife was very intelligent so she went to alexander and tied raki and said that u are my brother please leave me and my husband  and my kingdom i will compulsary tie the rakhi every year so the alexander came every year to india 
Raksha bandhan is a hindu festival that celebrates the love and duty between brother and sister. on raksha bandhan sister tie rakhi to his brother and in return brother gives gift to her. the brother gives promise to her sister to save her.