'Never smoke without fire'? That does imply that anyone facing an accusation is presumed guilty of something. It also goes against the legal premise of innocent until proven guilty. If you actually read Sion's book you will see that all the accusations came directly from Lois after she had been fed the report by Surrey police. Not before. There were no independent witnesses. Not only were there the other girls living in the house, but they also went through several live in nannies over the years. Lois was a social worker herself and Billie-Jo had regular sessions with her own social worker up until the end. But no complaints were ever raised.
Lois may well have known that her lies would have come out (the eldest girl flatly refuted them when they were first put to her). So she did her best to keep them out of giving any evidence in the original trial. There were tensions in the family due to Lois' career aspirations and wanting the family to move back to London. It was actually her who was a little strange due to an isolated upbringing. Sion describes how in one of their early dates before getting married, he realises she has no idea who characters such as Humphrey Bogart and Lauren Becall are.
Sion most definitely did not have a violent temper. He was a professional teacher on the brink of being promoted to head at his school on the basis of recommendations and his experience. He was well used to dealing with demanding children. Even the tent peg used as a murder weapon doesn't make sense if he used it. There were nearer weapons to hand that he could have used.