。
I could see my sister standing there.
Literal
Younger-sister [subject-が] there [at-に] exist [nominalizer-の] [subject-が] was-visible.
~のが見える ('can see that X') uses the nominalizer の to turn a clause into what is seen, with 見える (potential/involuntary 'can see'). いるのが — the の makes 'her being there' into a visible scene.