بسم الله الرحمن الرحيم
(Fatwa: 209/159/N=2/1439)
You should convince your sister and also try to change her thoughts through some pious and God-fearing lady. Also supplicate Allah for the same. I, too, pray wholeheartedly, may Allah guide her and distance her from that non-Muslim boy.
If possible, you or your mother should read the following wazifah with durood 11-11 times before and after daily 300 times. In-shaAllah, your sister shall distance herself from that non-Muslim boy soon.
يَا مُقَلِّبَ القُلُوبِ والأبْصَارِ ، يَا خَالِقَ اللَّيْلِ والنَّهَارِ ، يَا عَزِيْزُ ، يَا لَطِيْفُ ، يَا غَفَّارُ .
Allah knows Best!
Darul Ifta,
Darul Uloom Deoband