In my sister’s case, she allowed the perfect to be the enemy of the acceptable. She had a list longer than my arm of all the things the only guy she would deign to marry HAD to have. He had to be taller than her, make more money, be more educated, never been married, have no children, have the right kind of hobbies, be very good looking, dress the right way, buy her the right kind of gifts (without prompting), be a great cook, have the right career, be the perfect communicator, be great in bed, love cats, the list went on and on. As she got older, the pool of men that fit all her rules dwindled to nothing, and she wasn’t “settling.” Now she’s 50, in ill health, alone, and bitter about the few guys that came close to meeting her standards who ended their relationships with her (probably because they got tired of trying to be perfect enough).
Your poor sister sounds like she is incapable of emotional intimacy.