\indvdldef{Dudly}{Dudly}
\indvdldef{Petunia}{Petunia}[\matrilineal]

\indvdldef{Harry}{Harry}
\indvdldef{Lily}{Lily}[\matrilineal]

\sblngdef{sis}{Petunia,Lily}
\gensdef{twofam}{sis}{%
  {Petunia}{Dudly},%
  {Lily}{Harry}%
}

\begin{tabular}{l}
\fbox{\usebox{\sis}}\\
\\
\fbox{\usebox{\twofam}}
\end{tabular}