According to the Hindu Adoptions and Maintenance Act, 1956 there is no provision related to legally make someone your sister or brother. Only parents can adopt a child or a person who wants a sibling can tell their parents to adopt a child. Adoption of a married woman is not possible in India and hence you cannot make any legal relation between you and your friend whom you consider as a sister.
I would suggest that you should not go for a legal tie with your friend whom you consider as your sister. You should not listen to what other people are saying about you. A relationship to be fulfilled does not need any legal boundaries. If your family considers her as their daughter and you consider her as your sister then you do not need to prove this to anyone else. Even if you had any choice to legally make her your sister then it would have been waste of efforts. Your parents can adopt her if they want with her consent. Adoption leads to sharing all the legal formalities if she is adopted then she will have her right on your property as well. It is advisable that you maintain your relationship with her without legal boundaries.
Reference: Hindu Adoptions and Maintenance Act, 1956