Texas foreclosure laws tend to side with the lender more than the borrower. 20 days notice is all the borrower has to give, but especially these days my understanding Is most send multiple notices to try and work out a solution prior to the 20 day notice.
I'd suspect this isn't the first communication your brother's received from his lender. If he's not spoken with them, he should do so immediately. They may be able to provide options to work out a solution.
You are right that 20 days seems short - just today, the Dallas Morning News wrote an article about this:
Texas' fast foreclosure process hard on homeowners 
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Avoiding Foreclosure
