Family terms are set by the official program material and have been revised, so they must be read there directly. Do not assume they mirror the derivative rules used in the visa categories.
Ask the official page three specific questions
Hypothetical example: the couple have a son of twenty-four and a daughter of twenty. In the immigrant investment route only unmarried children under twenty-one are derivatives, so the son would need a basis of his own. It would be a mistake to carry that rule across by analogy, and an equal mistake to assume the opposite. The official page and FAQ state how family members are handled and how they are priced, and that is the only answer worth acting on. Put three questions to the official material: who may be included, what is payable for each person, and what happens if one family member is later found inadmissible. Where the material does not answer one of them, treat it as open, and do not let anyone give up employment or housing on an assumption.
Hypothetical example: a percussion-instrument importer wants to include a spouse, an adult child at home, and a younger child studying abroad. The first review should list every person separately with citizenship, relationship evidence, date of birth, planned participation, and immigration history. Civil certificates settle family facts, while the current official terms settle treatment and price. Obtain answers before anyone changes work or housing. A common error is importing derivative rules from another category or assuming financial dependence controls inclusion when the governing official material may say something different.