Is it permissible for me to call a marriageable woman by saying ‘O, my sister’ instead of uttering her name, or should I call her by her name while saying ‘my sister’?
All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.
There is no harm for a man to say to a woman ‘O sister’ or 'my sister such and such’ and ask her to give him something or to do something provided he is not in seclusion with her, while observing religious obligations in speaking to a marriageable woman.
Allaah Knows best.
You can search for fatwa through many choices