Here is the official answer, straight from US CBP website.
Who is eligible to file a joint or family declaration on a CBP Form 6059B?
A joint or family declaration can be filed on a CBP Form 6059B by persons who meet the following three conditions:
1. Are related by blood, marriage, or adoption;
2. Lived together in one household at their last permanent residence; and
3. Intend to live together in one household after their arrival in the United States.
This is in accordance with
19CFR148.34(b).
For more information about the Declaration Form 6059B, including a sample and instructions on how to complete the form click
here.
Effective January 17, 2014, Customs and Border Protection (CBP) broadened the definition of "members of a family residing in one household" to include long-term same-sex couples and other domestic relationships. Visit
cbp.gov/travel for more information.
"Domestic relationship" would be defined to include:
- foster children, stepchildren, half-siblings, legal wards, other dependents, and individuals with an in loco parentis or guardianship relationship;
- two adults who are in a committed relationship including, but not limited to, long-term companions and couples in civil unions or domestic partnerships where the partners are financially interdependent, and are not married to, or a partner of, anyone else.
https://help.cbp.gov/app/answers/de...int-or-family-declaration-on-a-cbp-form-6059b