The current official FAQ permits a spouse and unmarried children under twenty-one to join the cardholder if included in the initial application, with a separate processing fee and individual gift for each. Re-check those terms before any payment.
Three questions, answered from the official page
Hypothetical example: the geologist has a son of twenty-seven and an unmarried daughter of nineteen. Under the current official FAQ, the son does not fit the stated family category and needs an independent basis, while the daughter may be included in the initial application. Each included family member is subject to a separate DHS processing fee and individual gift and must independently satisfy admissibility and the applicable visa requirements. Record the ages, marital status and intended applicants, then confirm the current official terms, charges and visa availability immediately before any fee or gift is paid because the framework may change.
The second holds the household's own facts: each adult's citizenship, current employment and any business interests, the assets available and their sources, each child's age, and what the family is actually trying to achieve. An adviser engaged by the principal is not thereby acting for a spouse or an adult child, and interests can genuinely diverge where one person's history raises an admissibility question that would affect the timing of everyone else's case.