@John *********
it's not possible to be a dependant of a daughter who is older than 18 years. A mother can get a dependent Non-O visa for a child that is going to school in Thailand on a Non-Imm-ED visa, but when this child is over 18 years old, she can't be on a dependant visa any more. The daugther is on a Non-Imm-B visa and working in Thailand, and I have never heard that a mother can get a dependant visa based on a daughter working in Thailand