If the unit is rent stabilized, follow the link below regarding the renewal issue...see an attorney if necessary.
Usually when you sign a lease you have to disclose who will be residing at the property and leases may contain a clause about visitors staying at your home. If your brother is a resident the landlord can request that he leaves or you may be in violation of your lease. If you have concrens you need to discuss this with an attorney who can give you legal advice pertaining to this matter.
All the best,
In the area where I have rentals an eviction can happen with a 30 day notice and the magistrate is not very forgiving on additional tenents. The jurisdiction next to mine where I have a friend that owns properties the magistrate tells the landlord let the lease run and just don't renew them.