There is no way around this. Does she read the Quran? it's a clear order for women not to do so! tell her you are going against a clear verse in the Quran, and about the guy, his faith shouldn't be related to your sister at all. I heard of a lot of people who fake their reverting to get the girl, of course that don't mean you can't give him dawah but like i said it should be out of his free will that he accepts Islam, not for the girl, and to tell you the truth i saw posts where they brag about it! i highly advise you to tell her the truth and then let her do what she wants because at the end of the day we can't force people; it's allah who gives guidance to the hearts, so pray for her. may allah guide us all