1.Your sister can lodge complaint u/s 498A IPC against her husband.
2. She can file case for maintenance ,damages and compensation also under PWDV Act case.
3. She can file suit for divorce on the ground of adultery also.
4. So she does have these options.However all these actions would lead to breaking of marriage. Hence it is advisable if a final effort can be taken to reform her husband for the sake of the children.