Perhaps a bit of truth in each lie/line: she went, she fell, not coming back...
No one says that to a child/ren of any age about their mum, ever, under any circumstances)
Only a seasoned psychologist might know why he did and why Olivia repeated it... (seems this brother and sister lack or share some twist/darkness)
It really is right out there WRONG to put this in the three girls minds.
Then Olivia spends more alone time with them driving them to school - the mind boggles what she said unsupervised.
Then the Principal calls GBC as police sit down with him. Did the girls react once free of their aunty (and GBC hand up signals for silence!) finally free and feeling safe enough to react and to cry - at school. (?)
IMO