Love issues with already married man

Dear sir, My sister loves one person for the last 3.5 years . that guy is already married and have 3 years old child. We don’t know anything about his family details. When we started to enquire about his famiily deatils he itself came to home and said he has a family with girl child and everything my sister knows before itself. Only beacuse of her support and statement he proceed to love her. We asked with my sister that wheater is it all true, she said yes its all true and i knows before itself but i love him that much as she said. We don’t want to allow her to marry him. We don’t like him also. Now he is giving pressure for us to marry her. He is saying he lost his marriage life for the past 3.5 years only because of my sister wordings like he said. He is saying he spend more money and time for my sister for these days. We don’t know how to proceed with this. If we give complain means my sister will says she knows everything then only she encrouge him to proceed to start life with her. If she says like that what we need to do. Is there any possible way for giving case under bigamy against him onbehalf of my sister and if she is not accepted for this then what step we need to take further.