\indvdldef{Petunia}{Petunia}
\indvdldef[\blank]{Vernon}{Vernon Dursley}[\haschild]
\mrrgdef{Dursleys}{Vernon}{Petunia}{}

\newsavebox{\boxA}
\savebox{\boxA}{\hbox{Petunia}}
\indvdldef{Lily}{\hbox to \wd\boxA{Lily}}
\indvdldef[\blank]{James}{James Potter}[\haschild]
\mrrgdef{Potters}{}{Lily}{James}[\dimexpr\wd\Vernon - \wd\James\relax]

\sblngdef{sis}{Dursleys,Potters}
\fbox{\usebox{\sis}}