Well, it's already well established that they could, so trying to stuff that genie back into it's bottle via a BS cop out or idiot ball distribution would be worse. No need to force drama just for it's own sake.
To be fair, it's not like she only showed herself to the youngest, but that it was only the youngest that could see her at all. Now we only need to know what was the thing that attuned our MC towards the supernatural, as it now is confirmed to have happened a long time ago.