A de facto family is best described as:

A de facto family is defined as a situation where two adults are living together in a domestic relationship without being legally married. This arrangement typically involves a mutual commitment to a shared life and may include aspects of emotional and financial interdependence, similar to that of a married couple. It recognizes the validity of relationships that may not have formal legal status but function as a family unit in practice.

The other descriptions do not align with the definition of a de facto family. A family that has undergone legal adoption refers specifically to legally recognized relationships, which is not applicable to de facto families. A family consisting solely of siblings does not fulfill the criteria of a de facto relationship, as it lacks the adult romantic partnership component. Lastly, a family that has biological parents only addresses traditional family structures and excludes other forms of familial relationships that are acknowledged under the concept of de facto arrangements. Therefore, the identification of a de facto family as two adults living together without being married is the most accurate characterization.