Off to go find her brother...

(We'll just refer to the sister as Alice, and the brother as Jack)

While the slime did have full control of the sister's body, the slime actually did allow for Alice to still think of her own accord, and the slime was able to take note of everything that Alice was thinking about, like an outside observer.

It would use this ability to read Alice's thoughts as a tool to mimic her behavior more genuinely.

The slime wandered about the house, not knowing that there was yet another human being in there that it could learn much from.

And it was only matter of time before...