Answer
(Fatwa: 250/165/L=1434)
If your bahnoi (sister’s husband) is missing in a way that it is not known whether he is dead or alive then your sister should file her case in local Shariah Panchayat. After investigation if the Panchayat passes its decision as per the principle mentioned in al-Hilah al-Najizah that the missing person is dead then your sister shall be entitled to do nikah anywhere else after passing the iddah of death. Before revoking the nikah she shall remain in the nikah of the missing person and it shall not be lawful for her to marry anyone else.
Allah (Subhana Wa Ta’ala) knows Best
Darul Ifta,
Darul Uloom Deoband