All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
Our advice to your brother is that he refrains from getting married to that woman even if his father did not object, since he is not pleased with her. By refraining from this marriage, he will fulfill the highly recommended obedience to his father.
He should be sure that if he leaves that woman for the sake of pleasing his father, which is in turn an obedience to Allah, The Almighty, then Allah will compensate him with what is better. This is according to the Hadeeth narrated by Ahmad in which he reports Abu Qataadah and Abu Ad-Dahma' as having said: “We frequently visited the Sacred House (Ka'bah). One day we passed by a Bedouin's house. The Bedouin said: ‘The Prophet held my hand and began to teach me from what Allah Almighty has taught him. He told me that there is nothing that you leave in fear of Allah, The Almighty, except that Allah will reward you with something better.’” [This Hadeeth is authentic according to Shaykh Shu'ayb Al-Arna'oot]
Allah knows best.