U/S 125 CRPC, Your Sister in Law has all the rights to claim the maintenance from Her Husband.
The Marrige certificates or any other Proofs can be submitted to court on demand.
Sometimes the court may not give much weightage to the Marrige Registration certificate, In India, Still most of the people do not get there marrige registered. A Marrige card or Marrige Photograph is sufficeint proof to conclude marrige, Unless and Until Prove contrary.
For 498 A apply for a Bail at earliest. Otherwise, this will haunt you and your family for ever.
If you have good proof that the case is false, You can move the high court for Quashing U/S 482 of CRPC or you can move High court in Writ Petition Under Article 226 to stay or Quash the FIR.
Please note once the maintanence is granted U/S 125 CRPC, your brother will have to pay it, till the time your Sister in law gets remarried.