Each investor's own spouse and unmarried children under 21 may qualify as derivatives of that investor's petition. An adult sibling, parent or married child is not covered by a relative's case and would need an independent basis. Count the full number of people the group intends to move before anyone treats one petition as covering an extended household.
List every intended migrant against a petition
Write out the whole group as individuals and mark which petition each person would travel on. Names without a petition beside them are the finding: an adult child, a parent, or a partner who is not a spouse. Ask counsel early whether a second investor in the family would change that picture, since two petitions cover two sets of dependants.
Ages matter too — a child close to 21 may need the sequence arranged around them. Ask one further question of that list: what each person's status would be if their own case were delayed while others proceeded. A family that has assumed it travels together will not have thought about a household split by an evidence request affecting one participant, and the arrangements that make such a split tolerable, such as housing, schooling and healthcare on both sides, take time to put in place.
Record each family member's country of birth as well, since chargeability generally follows birthplace rather than residence or passport, and relatives do not necessarily share the same one.