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

\indvdldef{Harry}{Harry}
\newsavebox{\boxA}
\newsavebox{\boxB}
\savebox{\boxA}{\hbox{Petunia}}
\savebox{\boxB}{\hbox{Lily}}
\indvdldef{Lily}{Lily}[\matrilineal][\dimexpr\wd\boxA - \wd\boxB\relax]
\pcdef{Potters}{Lily}{Harry}

\sblngdef{sis}{Dursleys,Potters}

\begin{tabular}{l}
\fbox{\usebox{\Dursleys}}\\
\fbox{\usebox{\Potters}}\\
\\
\fbox{\usebox{\sis}}
\end{tabular}