This paper is in five main parts. The first introduces membershipcategorisation analysis (MCA) as originally outlined by Harvey Sacks and, here,as a possible extension of semiotic analysis. MCA is broadly a contribution todiscourse analysis in general and to conversation analysis in particular. Theapproach concerns membership categorisation devices such as family, the categoriesthey can contain such as ‘mother’, ‘father’, ‘child’, etc. and the categoryboundactivities or predicates commonsensically attachable to such categories.The second section looks at the legal background to family law in Australia andshows that its basic assumption is, by and large and with some exceptions, towork from categories (what people are) rather than from predicates (what theyin fact do). In the third section, we examine a particular Family Court case (RePatrick) which highlights the contestation between these approaches. Followingthis, we examine some recent shifts in the Australian states and territories towardsmore predicationally-based legislation and argue for their coherence incontemporary society and its increasingly flexible conceptions of what mayconstitute a family. Finally, we return to the question of semiotics generally andmake a case for our MCA-based distinctions as contributions to a possiblesemiotics of law.