The Prophet ﷺ said, "None of you should propose marriage over the proposal of his brother."
أخبرني هارون بن عبد الله قال حدثنا معن قال حدثنا مالك ح والحارث بن مسكين قراءة عليه وأنا أسمع عن ابن القاسم قال حدثني مالك عن محمد بن يحيى بن حبان عن الأعرج
أن النبي ﷺ قال لا يخطب أحدكم على خطبة أخيه
[Its isnad is sahih]. Ahmad narrated it: 2/462 from the hadith of Malik through this route, and it is in Muwatta (Yahya): 2/523, and in Kubra, no: 5355, and Bukhari narrated it in the Book of Marriage, chapter, "No one should propose to a woman already proposed to by his brother until..." etc, no: 5143, from the hadith of Jafar bin Rabia from Araj through this route, at length.