I think I know how your sister could get to such a spot in her life. I don't know her story, but here's mine.

My heart got broken at 22 - first love left town for just three months supposedly for business, we were not engaged but had an understanding. He returned with a red-headed West Texas fiancée in tow. No explanations no apologies. Again my heart got broken at 25 - reconnected with somebody I knew at age 19 when I was 25. We were going to get married. When the day came he said he decided to re-marry his ex-wife. I changed jobs just to break up my daily routine and forget him. I got involved with a guy at age 26 who said he broke up with the last girl because she was too dependent on him. Within a year he was complaining that I was too independent. After two years the only reason I kept him around was the great sex. I broke up with him at age 28. The men who came afterwards were just one kick in the teeth after another. Finally at age 31 I had had it up to here. I decided I wanted to go back to school, concentrate on my education, and get my MS. I got two of them actually. I had a tubal ligation at age 30 because I could no longer take the pill, did not want to risk pregnancy, and even more did not want to risk reproducing someone who would just be destined to feel the same pain I had up to that point.

From that point forward until age 38 I would find guys at work to hook up with for sex or an occasional evening out - it was easy, I'm an engineer and there are not that many women in my workplace, never have been. At age 38 premenopause hit and took away the only reason I had left for wanting a man around - a sex drive. Now at age 55 men are only good for home and auto repair as far as I can see.

Some background - I was always the ugly sister. When my two younger sisters married, nobody ever acted as though that was strange, or questioned my sexual orientation, or offered to set me up. I guess they actually LIKED their single male friends!

Again, not your sister's story, just mine. But I can see how somebody could get burned to the point that they just don't care anymore.