Sahih Muslim Book 8, Hadith Number 3287.

Chapter: Prohibition of one’s making the proposal of marriage when it has already been made by one’s brother until he permits or gives her up.

Ibn Umar (Allah be pleased with them) reported Allah’s Apostle (May peace be upon him) as having said this: A person should not enter into a transaction when his brother (had already entered into but not finalised), and he should not make proposal of marriage upon the proposal already made by his brother, until he permits it.