Start collecting evidence of any sort to prove the following:
1. Marriage was simple and no exchange or demand of dowry was made or entertained.
2. The couple were having problems from a long time and efforts were made to reconcile their differences. Start noting all the dates of trying to mediate between them and the result of the efforts including details of conversations, video or audio recordings if possible.
3. Emphasize that they were living with her parents and so therefore no case for dowry demands were possible.
4. Get sensible legal advise and most important don’t panic. No real case is made out at this stage and harassment is a long way off. Please remember that to harass somebody, we always have to first harass ourselves.
5. If the marriage is truly over, then ask your brother-in-law to file for divorce now. Any steps she takes after that to initiate Sec.498A case will then be viewed as harassment.