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

\indvdldef{Lily}{Lily}
\indvdldef[\blank]{James}{James Potter}[\haschild]
\mrrgdef{Potters}{James}{Lily}{}

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