Shock even at the age of six years old or slightly young can cause memory block.
The fact William has never been found his older sister has blocked it from her mind as the truma and guilt she was the last one to see him can affect her IMO. If it correct she help file charges against her Foster Carers, I taken it as this as one angry girl lashing out(or it could be be correct it happened)
Sorry again bring up my back grounding, but I struggle to remember events when I was six so does my older siblings, my clearest memories I have starts March 1974, after our Mother left us in a Children home and walked away. Years of fear etc. Years before this it is fragments of memories which I starting to put together along with paper trail and help of Goggle Earth ( my Mother left misinformation with the Welfere.)
I mean, thanks to Webseulths, I found the house we lived in a short while before going to Children Home in Westren Australia . We remembered the house and surroundings , not the street or suburb. Myself and siblings were left confused when reading years later reading our welfare files we knew the street and Suburb given in our files was not correct as she said the street was in Highgate. Due to Members of Websleuth discussion about the Claremont Killings, long before the arrest was made, at the time I was an on and off lurker, one of the member put up the original newspaper clippings and photos where one of victim last confirm sighting. showed to siblings, the bridge to above Railway Station, telephone box the area we remembered it we used Goggle Earth within minutes we found the house in Claremont where we had lived earlier. You would think at the age of six years I would remembered the address.
Knowing things and blocking it out can lead to anger, confusion and frustration, but it the mind protecting itself, IMO William's sister would be feeling this very strongly.