How to deal with the matter if someone tries to commit suicide?

My sister was 17 and going to turn 18 she was in a relationship with a guy and they had physical relationship for 3 years and he told her he's going to marry her when we found out and asked him about that he said he'll marry her but then he told her he's not going to marry her and he like other girl but still continued to talk to her and because of that my sister drank poison and a complaint was made then the police asked him and his mother to come they came and told us after she turned 18 he will marry her and the police also said that they can't do anything because she's under 18 and if we want they'll arrest him under POCSO but my sister said she just wants to marry him nothing else if not she is going to die what to do now