I feel very sad for Mahi. But the even sad thing is both Selvi and Arasi are both right in their places.
Arasi is a police officer. She has to do her job, and if every police officer gets emotionally involved in each case they deal with and get sentimental then nothing will get done. Each officer will get selfish for their own respective reasons and then who will think of protecting society in general. So Arasi is right in a way for being the iron lady, she is thinking of how future such crimes can be prevented. Maybe she is just seeing Mahi as another girl. But one thing I understood was why she couldnt get that other girl Kavitha's help, remember she shot all those who kidnapped her so she already punished them and forced the IAS guy at gun point to sign the shooting order. She can get into some big trouble if the IAS guy opens his mouth, and that is why she cant go digging into that specific case. In some odd way she is asking Mahi help because she considers Selvi her daughter deep down and knows that she can convince her own daughter to support her and she somehow sees a little bit of herself in her own daughter and knows Selvi like her will ultimately fight for justice, but the other girls parents will not listen to Arasi.
Selvi if this happened to her specifically she would go to court, but her sister Mahi is not as strong as she is. She needs to fulfill her duty as an older sister, when Mahi is begging her not to send her away. How can she not want to protect her sister wishes.
Mahi too the kind of person she is we cant expect much from her. And what happened was very tragic we cant expect her to punish the wrong doers when she is unable to find the courage to face it herself. I know we can all sit here all call her a dope for not looking at the bigger picture, but her character is very realistic I am sure there are many cases in india where girls like her dont speak up because they are scarred by the deep wounds, pressurized by family not to speak up, society in general plays a big part in ones psyche.