Dear Sir,
She cannot file 498A against you without her husband, she can file case against you along with your husband, If she files 498A case against you then you can file a Writ petition u/s 482 Cr.P.C to quash the case of 498A filed by your sister in law.
The following information may kindly be read:
The Supreme Court has said in a recent order that Section 498A of the IPC ( Dowry harassment charges) has "dubious place of pride amongst the provisions that are used as a weapon rather than a shield by disgruntled wives".
But the earlier fear of imminent and automatic arrest of in-laws in such cases has reduced now. In July, 2014, seeing the rampant misuse of the Section, the Supreme Court of India remarked that women were increasingly using the Section to harass their husbands and in-laws. The apex court restrained police from arresting the husband or his relatives on the mere lodging of a complaint under the Section. It required them to satisfy themselves about the necessity of arrest under the parameters set forth in section (41) of the IPC, which provides a checklist or conditions to be satisfied before arresting any person involved in a crime punishable with less than seven years of imprisonment.
The best suggestion would be to be cautious and have sufficient proof that she has indeed threatened to file a false dowry harassment case. It would be vital in countering her claims in front of police and courts and eventually be ridden of any guilty charges. You can Write a complaint to your nearest police station, detailing about blackmailing, her false allegations and behavior. And request in your complaint to make her stop the threats immediately.
The Police cannot arrest a person, without investigating the matter thoroughly and after collecting evidence only the Police can proceed. If it is later proved in the court that the wife had given a false complaint against the husband U/s.498A Domestic Violence Act, this itself could be used as a ground by the husband for obtaining divorce.