Is it permissible to marry a girl who was nursed by a foreign woman along with my brother, knowing that all scholars have issued fatwas permitting it, and how can one get rid of the devil's whispers concerning this matter?

If the woman's suckling with your brother was from an unrelated woman, then there is no impediment to your marriage to her, and there is no need for misgivings or doubts. Rely on Allah and do not pay attention to suspicions that have no legal value.