Divorce

As per the facts, your sister cannot perform the second marriage during the subsistence of the 1st marriage, because the second marriage during the subsistence of 1st marrige is an offence of Bigamy under IPC.

Because the marrige of 1st husband is not disscolved by way of divorce decree/ judgment by a competent.

1. your sister can file a contested divorce case on the grounds of cruelty, desertion, separation

2. in case, he doesn't appear, she will get the ex parte decree and can remarry after obtaining the decree and lapse of the appeal period

I completely agree with Shri Suneel Moudgil because the person wishing to remarry is a she.

I suggest men, " Don't marry; be happy."

if your sister wants to remain in the matrimonial relationship, then, try to settle the matter amicably involving elders/relatives/friends etc and ask her to start afresh

AND

if she wants to terminate the matrimonial knot, then, try to settle the matter amicably with the husband involving elders/relatives/friends etc and go for Mutual Consent Divorce which will be decided in 6-18 months, and your sister will be free to live life wishfully, 

don't forget to draft an MOU before approaching Mutual Divorce, which is the most essential and important part of a Divorce case,

some basic points to keep in mind and put in writing through an MOU before filing the petition for mutual consent divorce are custody of kids, distribution of assets (movable/immovable), quash/withdraw of pending litigation if any, Maintenance (present/future), Alimony, mode and time of making the payment, streedhan, future litigation, etc.

if failed, your sister can file/claim

1. 498a complaint (for causing mental, emotional, physical and monetary abuse,)

2. Domestic Violence case (for causing mental, emotional, physical and monetary abuse, etc)

3. she can claim residence, protection, compensation, maintenance,