A de facto family is best described as:

A de facto family is accurately described as two adults living together without being married. This definition captures the essence of a de facto relationship, which is characterized by a domestic partnership where the individuals are committed to one another and share a household, despite not having formalized their relationship through marriage.

In legal terms, de facto relationships can often provide similar rights and responsibilities as those enjoyed by married couples, particularly in matters of property, parenting, and financial support. This arrangement recognizes the social and emotional bonds that form between individuals, regardless of their marital status, emphasizing a broad and inclusive understanding of family structures beyond traditional definitions.

The other options describe different family dynamics that do not align with the definition of a de facto family. Legal adoption involves formalizing a parent-child relationship through legal means, which does not fit the characteristics of a de facto relationship. A family composed solely of siblings does not encompass the notion of a domestic partnership between two adults. Finally, a family consisting exclusively of biological parents does not address the larger spectrum of family forms, which include non-marital partnerships. Thus, the option representing two adults living together without marriage is the most accurate depiction of a de facto family.