I find it interesting that even when US changed his story and took the blame for the majority of Sara's injuries, he was adamant that he didn't bite Sara, and he denied covering her head with a hood or burning her with the iron. It is also likely that the iron would be used mainly if not exclusively by BB, so it is likely that she was responsible for the burns.
We know that neither he nor Malik were responsible for the bites because they agreed to have their teeth impressions tested whilst BB didn't. And we know from the neighbour's testimony that they frequently heard a woman's raised voice during the day and the cries of children/Sara, so clearly BB was involved in beating Sara even if the more vicious assaults were down to Sharif.
All in all, as others have said, every adult in Sara's life, not only let her down, but also treated her like a punch bag. It's likely that her mother also was violent otherwise why would they have given US custody particularly given his history of violence. The poor girl lived a life of terror and torture from the beginning to the awful end. I really hope that the verdicts and then the sentences reflect that.