How can one get rid of the obsessive thoughts of loving one's husband's brother and stop trying to communicate with him, while adhering to ذكر (remembrance of God), reading the Quran, and seeking forgiveness (استغفار)? And will this obsession last for a long time?

As long as you have repented from this dangerous path, then remain firm in your repentance and turn to your Lord. Do not give Satan any way to reach you, and sever all ties with this man. Among the greatest means of protection from returning to sin is sincere reliance on Allah, seeking His help, and trusting in Him to divert trials from the heart, along with staying away from the causes of temptation. And supplicate abundantly and insistently.