In your law Muslim women has minimum rights to take divorced as compare to Muslim boy.
Ask your sister to file the case of divorced against that boy under- Dissolution of Muslim Marriage Act,1939.
Grounds for decree for dissolution of marriage under this act as - \
u/s.(iv) that the husband has failed to perform, without reasonable cause, his marital obligations for
a period of three years;
u/s.(v) that the husband was impotent at the time of the marriage and continues to be so;
u/s.(viii) that the husband treats her with cruelty, that is to say-
(a) habitually assaults her or makes her life miserable by cruelty of conduct even if such
conduct does not amount to physical ill-treatment.