The prohibition of proposing marriage when one's brother has already proposed, unless he gives permission or gives up the idea Sahih Muslim 1414
Translation
'Uqba b. 'Amir said on the pulpit that Allah's Messenger (ﷺ) said
A believer is the brother of a believer, so it is not lawful for a believer to outbid his brother, and he should not propose an engagement when his brother has thus proposed until he gives it up.