The Man-Woman Relationship
If, as a woman, you are indulging in an intimate friendship with a man who is not pursuing you, you are accepting a cheap imitation of love. And by spending all your time with a guy who will never put a ring on your finger, you may miss a potential suitor.
If, as a man, you are spending large quantities of time with a woman, you may want to consider if perhaps the relationship is deserving of an upgrade to an intentional relationship that explores the possibility of matrimony. If not, do your sister the courtesy of making your stance clear, freeing her to be pursued by another man.