How can I save my sister from her cheating husband?

My sister got married in August. Her husband has always been isolating her in his home, coming late from work, not talking to her and sleeping in separate room.He's been abusing her mentally by blaming and gaslighting her thats she's the reason for their life being so hard (All she did is asking him to spend time with her). She had a panic attack and he almost didn't save her and wanted to go to work. He even aggravated it and stopped the car otw to hospital and asked her to get down. Then when going from one hospital to another in ambulance he wanted to come in his car and instantly lied that the ambulance didn't have space. He got in after the driver said we had space for him. We got to know he's doing all this because he's cheating and wants to dissolve this marriage to be with the other girl. He still hasn't registered the marriage and has been pushing it away till now. What can we do legally or by any means?