  Using the cohomology of the $G_2$-flag manifolds $G_2/U(2)_{\pm}$, and their
structure as a fiber bundle over the homogeneous space $G_2/SO(4)$, we compute
their Borel cohomology and the $\mathbb{Z}_2$ Fadell-Husseini index of such
fiber bundles, for the $\mathbb{Z}_2$ action given by complex conjugation.
  Considering the orthogonal complement of the tautological bundle $\gamma$
over $\widetilde{G}_{3}( \mathbb{R}^{7})$, we compute the $\mathbb{Z}_2$
Fadell-Husseini index of the pullback bundle of $s\gamma^{\perp}$ along the
composition of the embedding between $G_2/SO(4)$ and $\widetilde{G}_{3}(
\mathbb{R}^{7})$, and the fiber bundle $ G_2/U(2)_{\pm} \to G_2/SO(4)$. Here
$s\gamma^{\perp}$ means the associated sphere bundle of the orthogonal bundle
$\gamma^{\perp}$. Furthermore, we derive a general formula for the $n$-fold
product bundle $(s\gamma^{\perp})^n$ for which we make the same computations.
