Your sister is vulnerable in her own situation and advising her to stop this relationship is futile. If you think you wont be nagging by taking some examples of such marriages between different sects of religion that have failed or are facing a lot of hardship, maybe you can give her such examples. You could also advice her lovingly to seek advice from a learned member of the community or an elder or close aunt or even a good friend of hers. In the meantime you and your parents can pray and tell her without pressuring her that she should really think well before making this big decision which would effect her and even her generation ahead.
If the person she's chosen is a good man, he might change also due to her, one may never know, although it is quite rare but it is possible. To marry a sunni is not forbidden if they don't have ill feelings towards the Holy Prophet and his family. If they do, then they would not be sunni but Naasibi.
In the end, maybe you can tell her to take out an istekhara and see what Allah has to say regarding the marriage. After all she will be confused when you all are warning her of the consequences so it befits that you take out an istekhara after she makes the intention and wants it to be done.
Every kind of prayer can be done to ask Allah for good outcomes eg, isteghfaar, salawats, reciting Hadithe kisa, a forty day amaal, etc.
InshAllah she will make the right decision.