Vacharaeson Vivacharawongse, 42, arrived in Thailand last week and visited a child care centre and took part in religious ceremonies at a Bangkok temple.
Their visits come at a fraught time for the royal family, with the monarch's eldest daughter in a coma since December.
Vacharaesorn and Chakriwat are the second and third of four sons of King Vajiralongkorn's second wife, Sujarinee Vivacharawongse, a former actress whom the then-crown prince divorced in 1996.
Vacharaesorn's younger sister was later re-embraced by the royal family and given the royal title Princess Sirivannavari Nariratana.
Reporting by Juarawee Kittisilpa, Napat Wesshasartar and Chayut Setboonsarng; Editing by Nick MacfieOur Standards: The Thomson Reuters Trust Principles.
Persons:
Chakriwat, Maha Vajiralongkorn's, Read, Maha Vajiralongkorn, Vacharaeson Vivacharawongse, Vacharaeson, Vacharaesorn, King Vajiralongkorn's, Sujarinee, Sirivannavari Nariratana, Juarawee Kittisilpa, Napat, Nick Macfie
Organizations:
Thomson
Locations:
United States, Thailand, BANGKOK, Thai, Bangkok