Well, you do not own your sister, so best you can do is voice your concern for her and leave it at that. don't get in the middle of it, because if you do and it doesn't work out, she will blame you and will be able to rationalize herself into a similar situation. PLUS you don't know his intentions and she needs to figure this stuff out on her own, it's part of growing up and figuring out what you're looking for in a companion.
I have a friend who started dating a man 15 years older. 6 years later they are married with a child. If I followed your logic I should have said, "he drives a land rover, hes just looking to hit it and quit it" and see how terrible of advice that would have been.
Women are usually attracted to men that have money/status and are mature, attractive, and confident.
Men are usually attracted to women that are young, attractive, and rational.
Is it really weird to not want your sister to get hurt guys I mean really? How isn't what your close family does your business? You can't force it but you sure as hell can look out for them. Its like most of you don't have anything more than a brain stem.