The information above is general simply because your initial post did not give the state in which you are located (I provide as much information as possible because some customers do not return or do not answer follow up questions and I want to help by providing as much as I can with the information given).
In New York, this eviction is going to be classified and treated as a "holdover petition" (there are two basic types of eviction in NYS - Non-payment, and holdover). Holdover is the more complex one, and a local attorney is recommended (but again, not required - your brother and his wife can represent themselves if they choose to do so).
The basic defense that you have is already identified above, the only thing more specific is the exact process (so the resources available to your brother and his wife to assist them in defending the action).
The first, and most critical thing to do, is for your brother and his wife to file their "Answer" to the complaint. If they do not file an "answer" the landlord will win by default. In New York, it is possible to file an "oral answer" by simply walking into the housing court clerk's office and stating your answer, a written answer is preferred simply because it helps ensure that there is no confusion, or misunderstanding, it also allows you to reference any necessary documents (such as the POA agreement authorizing you to enter into lease agreements on behalf of the tenants).
In New York, the Courts have a helpful website that discusses in much greater detail how to file an answer and what exactly that does (basically it allows the tenants (your brother and his wife) to tell their side (they have a valid lease, it was signed by their duly authorized POA, and their POA (you) are prepared to testify at the hearing (understand these hearings are usually a single event, so be prepared to accompany them to the first court hearing).
The problem that your brother and his wife are going to run into is that the NYS form answers to not conform to this defense - this means that your brother and his wife must draft (write) their own answer. This is where going to the local law library is important. (This is still a relatively easy matter, and shouldn't be a complex answer (the issue is very straight forward, as summarized above in a single sentence), but it still needs to be written up and filed within the time for filing)
All New York Housing Courts have assistance centers as well - this may be a good place to visit in lieu of the law library to help putting together an appropriate document (they cannot give legal advice, but they can help with formatting and appropriate cover papers).
There is a legal aid for New York State (so a much larger state organization) that provides general information and guidance, http://www.lawhelpny.org/issues/housing/eviction?channel=know-your-rights&category=preparing-for-court&location=all
If you want a more specific overview (so more detailed and more specific to NY State) than what I provided above as to what happens each step of the way, I would recommend the following link, it is put together by Legal Aid of Western New York, and has additional resources that may be helpful as well: http://www.lawny.org/index.php/housing-self-help-141/housing-and-eviction-self-help-142/48-general-eviction-information