\documentclass{article}
\usepackage{istgame}
\begin{document}
\begin{istgame}
\xtdistance{15mm}{30mm}
\istroot(0)(0,0){B\textsubscript{1}} % root as (0) at (0,0)
\istb [above right]{P\textsubscript{1}}% endpoint will be (0-1)
% \istb % endpoint will be (0-2), automatically
% \istb % endpoint will be (0-3), automatically
\endist % end of simple (parent-child) structure
\istroot(c)(0-1) % names the subroot (c) at (0-3)
\istb % endpoint will be (c-1)
\istb % endpoint will be (c-2)
\endist
\istroot(d)(c-2)
\istb
\istb
\endist
\istroot(e)(d-2)
\istb
\istb
\endist
\istroot(f)[right=5cm of c-2]{B\textsubscript{1}}
\istb
\istb
\endist
\istroot(g)(f-2){B\textsubscript{1}}
\istb
\istb
\endist
\xtInfoset(e)(g){}
\end{istgame}
\end{document}