Your sister cannot remarry until her first marriage is dissolved by the decision of court as polyandry is prohibited by law.
In case your sister's husband is missing/non traceable for seven years or more then she can remarry without waiting for the decree of divorce as it is considered as 'presumption of death' a ground of divorce U/S 13(b), HMA, where in, the person is presumed to have died, if the family or the friends of that person do not hear any news about the person for seven years.
It is considered as the valid ground for divorce, but the burden of proof is on the person who demands the divorce.
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