The Messenger of Allah ﷺ forbade a man to propose marriage to a woman already engaged to his brother, unless the first suitor withdraws his proposal or grants him permission.
حدثنا أبو اليمان أخبرنا شعيب أخبرنا نافع أن
نهى رسول الله ﷺ أن يخطب الرجل على خطبة أخيه حتى يدعها الذي خطبها أول مرة أو يأذن له
Its isnad is sahih, and its meaning has already occurred repeatedly, most recently at 6034, but with the addition, "until he leaves it," which has not occurred before. Bukhari related it, 9:170,171, through the route of Ibn Jurayj, from Nafi, from Ibn Umar, "The Prophet ﷺ forbade that one of you should undercut another's sale, and that a man should propose to a woman already proposed to by his brother, until the previous suitor withdraws or gives him permission."