Seek Advice For Mental Harassment

sir my sister is married to an officer in indian air force for two years and have a daughter...we gave them what they demanded...he married my sister with his and his parents consent...but now he wants to leave her...her mother in law tortures her mentally and comments on her looks and shape...and keep saying that they will divorce her all the time but she wants to live with her..plz advise me about rules of marriage in indian air force and what should i do