Is it permissible for a committed young man and woman to consider each other as his "sister" or "a part of me" and express that with the word "habibati" (my darling/beloved)?

A man’s saying to a woman, "You are my beloved" or "You are a part of me," or a woman’s saying such phrases to a man, constitutes forbidden flirtation between non-mahrams. Allah has prohibited communication between the sexes except out of necessity and within the bounds of propriety. Allah Almighty says: "And when you ask [his wives] for something, ask them from behind a partition. That is purer for your hearts and their hearts," and "Then do not be soft in speech [to men], lest he in whose heart is disease should covet, but speak with appropriate words." These phrases are not among the "appropriate words." Religious commitment does not permit the forbidden; rather, it increases the ugliness of disobedience. One must avoid these phrases and limit expression to what necessity dictates, while lowering the gaze and guarding the limbs.