The Messenger of Allah ﷺ said, "None of you should propose to a woman who is already engaged to his brother."In another narration, he added, "...until the previous suitor withdraws or gives him permission."
آن رسول الله ﷺ قال: لا يخطب أحدكم على خطبة أخيه. وزاد في رواية: حتى يترك الخاطب قبله أو يأذن له الخاطب.
Malik narrated it in the Book of Nikah (2), from Nafi, from Abdullah bin Umar, and he related it.Bukhari narrated it in the Book of Nikah (5142), through Ibn Jurayj, and the addition mentioned belongs to him.And Muslim narrated it in the Book of Nikah (1412), through Layth (that is, Ibn Saad), Ubaydullah (that is, Ibn Umar), and Ayub, all four of them from Nafi, with the same wording, similarly, and he added at the beginning the prohibition against a man outbidding his brother in a sale.