Personally, I don't put any weight on William's sister's evidence.
In my experience and observation, a 4 year old can have ideas put into their heads by adults pretty easily and, years later, after being told a story about an event many times, can come to believe that they were there at the time although they have no real memory of it.
For example: I recall her being quoted as saying "A bad man took William" or words to that effect. It's my opinion that she did not witness any such thing but was told that by her fosters and came to believe it.
Yes Kids are easily manipulated and expected to hold mum and dads narrative about stuff.
The influence of your parents are all consuming to little people.
as you age and grow in confidence, you push back and TRUTH.... as inconvenient as it may be, starts purging itself.
I am inclined to see her developing her own conscience coming into adulthood and not being afraid to talk.
She has her own memories.
She is the true living victim of this situation and circumstance.
I hope she is living well.
moo