بسم الله الرحمن الرحيم
It is not lawful to get your sister married with a Christian boy rather it is haram. Hence you should not take any such step. As far as the matter of remarrying her with the new Muslim is concerned then in this case we cannot say anything until it is not known how the relation of both came to an end as well whether that boy follows Islam completely or not. You would better go to Darul Uloom Hyderabad and tell them the complete situation to know the ruling.
Allah knows Best!
Darul Uloom Deoband
This answer was collected from the official ifta website of Darul Uloom Deoband in India.