First of all, what do you feel is the right approach? You know your sister much better than he does. How is she likely to respond by your telling her your feelings about this man?

I think that it might also be a matter for your Mother to discuss with her, as her word may have more authority with your sister. Women have a way of cutting to the chase when it comes to discussing something like this. Men are often too soft on them to get to the point.