The Messenger of Allah ﷺ said, "None of you should propose marriage to a woman to whom his brother has already proposed."
قال قرأت على عبد الرحمن: مالك عن محمد بن يحيى بن حبان عن الأعرج
أن رسول الله ﷺ قال لا يخطب أحدكم على خطبة أخيه
Its isnad is sahih, and it has already occurred at 9861.