The prohibition of proposing marriage when one's brother has already proposed, unless he gives permission or gives up the idea
وَحَدَّثَنَا قُتَيْبَةُ بْنُ سَعِيدٍ، حَدَّثَنَا لَيْثٌ، ح وَحَدَّثَنَا ابْنُ رُمْحٍ، أَخْبَرَنَا اللَّيْثُ، عَنْ نَافِعٍ، عَنِ ابْنِ عُمَرَ، عَنِ النَّبِيِّ صلى الله عليه وسلم قَالَ " لاَ يَبِعْ بَعْضُكُمْ عَلَى بَيْعِ بَعْضٍ وَلاَ يَخْطُبْ بَعْضُكُمْ عَلَى خِطْبَةِ بَعْضٍ " .
Ibn Umar (Allah be pleased with them) reported Allah's Messenger (ﷺ) as having said this: None amongst you should outbid another in a transaction, nor should he make proposals of marriage upon the proposal made by someone else.