If the girl is a non-Mahram to you, then it is prohibited for you to have any contact with her.
A non-Mahram is literally translated as not prohibited. According to the Shari?ah, it refers to persons with whom marriage is not prohibited. Conversely, a Mahram refers to such persons with whom marriage is prohibited, for example, mother, sister and daughter, etc. If the girl in reference is a person with whom you can marry according to the Shari?ah, then you cannot regard her as your blood sister and have free contact with her. That is prohibited and a major sin.
and Allah Ta’ala Knows Best
Mufti Ebrahim Desai