There exist a fair amount of women who are into older men. I think the general populace consider the women who do have these things to have some sort of daddy issue, that they want to date their father in some sort of Electra complex.
Older men often have settled into a good career, they earn more money and are generally more successful. It is only natural that a woman is going to be interested in a man that they think could provide well for a family, no woman wants to marry into poverty.
Older men also often have a richness to their lives that many women would find appealing, they have a confidence that many men only develop when they settle into there thirties. All of this is very natural and not all that anti-social.
The bigger issue I see is why you are have an issue with who your sister dates at all. She is not a teenager anymore, why are you treating her like one?
Another question is why would you think a thirty year old man is more likely to hurt your sister than a 23 year-old?