Is it permissible to marry the younger, religious brother after the rejection of the older, irreligious brother, knowing that they live in the same building?

Undoubtedly, a woman is obligated to choose a righteous husband. A righteous man treats his wife well, unlike a wicked man. The questioner's rejection of the irreligious man is absolutely correct, and there is no blame upon her for accepting the suitor's brother if he is religious and has good character. The fact that the closer relative proposed to her first is of no consequence.