SHAFAQNA – The Grand Ayatollah Safi answered a question about unrelated man and woman becoming brother and sister by taking a vow.
Question: Can I take a brotherhood-sisterhood vow (Siqeh) with a non-Mahram girl, and (then we) behave like brother and sister?
The Grand Ayatollah Safi: No, brotherhood-sisterhood vow with non-Mahrams are not allowed according to religion. Permanent or temporary marriages with related conditions can lead to intimacy between a man and woman (Mahramiyyat).
Source: PERSIAN SHAFAQNA