It depends on the intellectual state of your sister. Sometime love disables rational thinking when it becomes a blind love. You should pray to Allah to avoid us and our beloved ones blind love. You should ask your sister whether she is ready to think about the future or not ready. She should think about her future after few years from such marriage when love emotions evaporate as usual. What will be the future of her children if any. Will she take the risk of their possible deviation from the way of Ahlul Bayt (AS)?

Simply tell her to study majority of such love marriages between persons from different faith backgrounds and take the lessons from thousands who suffered and still suffering but can not repair the damage.

When there is a possible harm on your or your future children faith or religion, you should avoid such risk.

If the man is open minded and ready to to read and understand and follow the Prophet and his Ahlul Bayt, then the matter is acceptable. Just promises are not enough, but real practical steps are required before the marriage.

 Wassalam.