Sister Love Affair

Hi, My sister is in love with his team lead in their company. After we knew about this our family approached to their family for marriage. We went to their house and then they kept on postponing marriage for 1 year and then came to our house for marriage discussions.Indirectly they said they were expecting Dowry and they left its been more than a year now and my sister and that boy both are still not moving ahead and are head bent to get married but boy's family is not supporting. Irrespective of what happens after marriage, I want legal immunity that my sister will not be my responsibility. I mean either financially or morally I'm not legally bound to help her in any case whatever happens after marriage. Is there any official document or can I get one such done and get it signed by my sister , it's her life, she has a right to it but she should also face the consequences of her decisions and not become a nuisance for me. Is there any official document or can I get one such done and get it signed by my sister so that whatever happens after their marriage she will not pester me or our parents in any manner if she's in any issue after marriage.