Case filed against a party by the accused himsef

My sister was married last year in December in Kanpur. she was mentally harassed since her marriage. during all this she became pregnant and delivered a premature baby boy in August this year. her in-laws admitted her in the hospital and left her. we brought her back to our hometown in Bareilly after giving a written application in the local police station of her in laws place stating the whole incident. later one day her husband came to our hometown and almost threatened us. then we filed a fir with the local police in bareilly. the matter went to settlement court to magistrate. after two unsuccessful hearings the third was scheduled on 25-10-2016. the other party din't report and informed that my sister's husband has chikungunea fever and he asked for another date on 15-11-2016 which the magistrate agreed. meanwhile we have received a registered letter form kanpur family court which says that we need to be present there on 15-11-2016 since a case under hindu marriage act 9 has been filed against my sister. now my question is when we have filed a case how can the other person file another case against the same issue? he is already a accused in our case? please advise