Is there any actual reason to assume this? Story can obviously go in many directions, but as of today, it seems that uncle molested his sister and that's it. Otherwise why would his maid let him go outside? Even if she doesn't believe that there is some extraterrestrial evil on the island, main character is still a weak kid, he could get easily lost or attacked by some wild animal.
Now i think about it, it's still weird that she didn't protest more. Unless she also is controlled by some outside force, we know that she acting different.
I don't think that it will be revealed what happened to her for a long time. It's the big mystery of the story.
Also, I wonder why there are no other kids in the village. It was mentioned once that there are no kids there and main character is the youngest person there.