Answer
(Fatwa: 907/574/L=1432)
If your sister did not commit any shirkiya and kufriya act after eloping, then she is a Muslim as usual, but she is sinful due to illegal relationship. She should do taubah and istighfar whole-heartedly. If she has not committed any shirkiya and kufriyah act her salah shall be acceptable. If she admits her mistake and asks for forgiveness, you should forgive her and bring her home and keep a close watch over her so that she could not commit such a wrong practice again.
Allah (Subhana Wa Ta’ala) knows Best
Darul Ifta,
Darul Uloom Deoband