If you cannot enter US before your immigrant visa expired, you need to contact the US Consulate or NVC to re-schedule an interview for a new visa. You may need to file DS 260 and apy the fee again. Alos, it may take a long time for you to get the interview.
So, your brother and his family had better come to US before the immigrtant visa expires. If they have no time to deal with business or something before they leave China, they can return to China a few days later after they arrive in US. The US immigrant officer at the port of enter will put a note (endorse) on the immigrant visa stamp on the passport when they enter US. This endorsed visa stamp is equal to the temporary green card valid for one year, so they can use it to travel to China before they get the plastic green card.
