How can one deal with a sister who refuses marriage, comes home late from work and perfumed, mistreats her parents, and does not accept advice, causing sadness and distress to the family?

We recommend frequent prayers for the sister's guidance, especially from the parents, as their prayers are answered. This is because Allah becomes indignant when a believer commits what He has forbidden, and the sister has done reprehensible things, among the most dangerous of which is mistreating her parents. In addition, she should be advised kindly, and the advisor should be someone whose words she is likely to heed. The best approach for dealing with her must be determined: either by boycotting her if that will benefit and deter her, or by encouraging and endearing oneself to her if that is more beneficial and effective.