Subspace of a homogeneous space.

Given two homegenous spaces $\frac{G}{H}$, $\frac{A}{B}$ with $A\subset B$ is there a way to prove that $\frac{A}{B}\subset \frac{G}{H}$ ie that $B\subset A\cap H$?

In particular I would like to show that

$$\dfrac{E_6}{Sp(1)U(5)}\subset\dfrac{E_7}{Sp(1)Spin(12)}$$

I am sure I am just missing out an easy answer that I just can't see.

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