It looks we don't need I157 for our parents over 45 years old according to below copy from Unitedstatesvisas.gov. Last year, my in laws got visa without that form:
A Supplemental Nonimmigrant Visa Application, Form DS-157 provides additional information about your travel plans. Submission of this completed form is required for all male applicants between 16-45 years of age. It is also required for all applicants from state sponsors of terrorism age 16 and over, irrespective of gender, without exception. Seven countries are now designated as state sponsors of terrorism, including North Korea, Cuba, Syria, Sudan, Iran, Iraq, and Libya. Select Special Processing Procedures to learn more. You should know that a consular officer may require any nonimmigrant visa applicant to complete this form. Here is Form, DS-157.