\begin{center}
\begin{tikzpicture}[yscale=.8]
\node (omega) at (0,0) {$\Omega$};
\draw (omega) -- (2,2) node[right] (D) {$A$};
\draw (omega) -- (2,-2) node[right] (DB) {$\overline{A}$};

\draw (D) -- (4,3) node[right] (R1) {$B$};
\draw (D) -- (4,1) node[right] (R1B) {$\overline{B}$};

\draw (DB) -- (4,-1) node[right] (R2) {$B$};
\draw (DB) -- (4,-3) node[right] (R2B) {$\overline{B}$};

\node[above left] at ($0.5*(omega)+0.5*(D)$) {$0,3$};
\node[below left] at ($0.5*(omega)+0.5*(DB)$) {$0,7$};
\node[above left] at ($0.5*(D)+0.5*(R1)$) {$0,9$};
\node[below left] at ($0.5*(D)+0.5*(R1B)$) {$0,1$};
\node[above left] at ($0.5*(DB)+0.5*(R2)$) {$0,25$};
\node[below left] at ($0.5*(DB)+0.5*(R2B)$) {$0,75$};
\end{tikzpicture}
\end{center}