How to sponsor a 19 years old Step Son?

My mother is a US citizen and lives in the US. She would like to sponsor her 19 years old step-child (my half-brother), who lives abroad. My mother is the first wife of my father, the step-son is from his second wife. My father never divorced my wife and she was still in a marital relationship when the step child was born. The step-child is now an orphan, whose mother passed away a couple of years after the birth and my father passed away last year. We would really like him to be with us since he is now all alone and treats my mother as a real mother. I believe there is a provision that allows a person to sponsor under 21 step-child , if the person was still married at the time of the birth. I will appreciate it if you can help me understand if this law can be applied in my case and my mother is able to sponsor her step-son. If we apply now and by the time the process is complete he is over 21, would he still be able to come?