@ Author I second Siv's opinion. Try collecting evidence and while file objections bring this to the court attention and request for call of records under sec 91 of CRPC. Ensure you file your objections strongly with all evidence. Please ensure the court would take into consideration all the factors in arriving at the Maintenance amount.
Lets say your brother's Salary is 1,00,000 and your sister in law is earning then court would definitely arrive at 1/3rd amount as maintenance the reason why the court will arrive at this formula is to ensure your SIL's maintains the same social status as your brother.
Lets say your brother is earning 25000 and your SIL Salary is 12 K, then the application will be dismissed.
If both of them have child born out of wedlock even though your SIL is earning good amount, she will be awarded the maintenance citing that child's responsibility has to assumed by husband and wife equally.
Now please do that math yourself to arrive where your brother's case fit in