Should one refrain from proposing to a girl due to her brother's past behavior (drinking alcohol and then quitting), or does this not affect the decision to propose?

If the girl is of good character and religious, then you should seek her, for the Prophet, peace and blessings be upon him, said: "...So win the one who is religious, you will prosper." Her brother's drinking of alcohol should not prevent you from her, because a person is not held accountable for the sin of another, as Allah Almighty said: "And no bearer of burdens will bear the burden of another." Especially if he has quit and repented, for Allah accepts repentance from His servants.