I think my 16 year old sister is dating a 26 year old?

You don't want to go at her making accusations or telling her what she is doing wrong because that will cause her to rebel. Just talk to her and tell her your concerns and that you only want the best for her. If she doesnt see the light and you fear for her safety you may have to inform your parents. She will be very upset at you but in the long run it may be for the best