Is it considered a sin to love a girl who is engaged to another man, while she desires to be a second wife to him and does not feel love towards her current fiancé?

You are advised to fear Allah, avoid mixing with non-mahram men, and adhere to the prescribed Islamic hijab. If you do not harbor feelings for your fiancé, be honest with him so he can annul the engagement. It is impermissible to think about your manager or become attached to him; you must divert your thoughts from him and avoid him to prevent falling into the malady of infatuation. You are advised to try and continue with your current religious fiancé, otherwise, annul the engagement to allow an opportunity for another suitor. You must despair of your manager, and if it necessitates leaving your job, then do so. You must marry a man of faith and good character immediately to safeguard yourself from the pitfalls of whims and Satan.