You do NOT need any letter or document from the OGS to invite a family member as a B2 visitor. If you wish to invite family members, other than your spouse or child, to visit the United States for a short time on a B2 visitor visa, you may provide them with the following:
Note that an individual in B1 or B2 status is routinely admitted to the US for a six month period.
Beginning November 29, 2016, travelers with a 10-year B1/B2, B1 or B2 visa in a People's Republic of China passport are required to enroll in the Electronic Visa Update System (EVUS) in order to be admitted to the United States in B1 or B2 status. Travelers using passports from Hong Kong SAR, Macau SAR, Taiwan, or any passport other than a People's Republic of China passport are not affected by the EVUS enrollment requirement. Please review the EVUS enrollment information for further details.