Is what the questioner did by rejecting the suitor, despite her brother's pressure and his family's interference, considered a mistake, and must she marry a young man for whom her heart has not been opened, despite her continued performance of the Istikhara prayer?

You are not obligated to marry, unless you fear falling into sin; in that case, you must accept a suitor to keep yourself chaste, because a woman is like a man in the obligation to marry if she fears falling into adultery. We advise you to accept this righteous suitor. After performing Istikhara (definition), a feeling of relief in the heart is not a condition; rather, you should proceed with the matter unless Allah turns it away from you.