What is the meaning of "does not believe" in the noble Hadith, "None of you believes until he loves for his brother what he loves for himself"? And does "does not believe" mean disbelief and apostasy from the religion, or does it indicate falling into sin while remaining Muslim? And is a second wife considered unbelieving if her marriage will negatively affect the first wife, especially if it causes the ruin of her home?

The Hadith "None of you believes until he loves for his brother what he loves for himself" does not refer to disbelief (kufr), but rather to the imperfection of faith. What is meant is the love of good, acts of obedience, and permissible things. A woman marrying a married man is neither a sin nor an injustice to his other wife. Emotional relationships between non-mahram men and women are forbidden, and exchanging words of love and flirtation is not permissible. In fact, some jurists have explicitly stated that it is forbidden to speak with a non-mahram woman without necessity. If someone loves a non-mahram woman, they are not allowed to exchange words with her. The permissible way is for him to propose to her. If marriage is not possible, then both of them must abandon this attachment and strive to get married to protect their chastity. You must cut off your relationship with this man. If he proposes marriage and he is of good character and religious commitment, then consult with wise people and seek Allah's guidance (istikharah).