It crossed my mind that perhaps, due to the frequency of U's beatings/anger etc, towards Sara (I mean, it can't have been pleasant, living in that house and knowing that another "episode" was always just around the corner) possibly Beinash grew frustrated with Sara and then began to abuse her too to try and prevent some of U's rages.
Almost as if she wanted Sara to just stop doing things to "wind her father up" (I know Sara wasn't doing anything, in any way, to cause this abuse, I'm just saying how abusers think and the anxiety they instill. I too have been a victim of DV and control, and you find yourself tiptoeing around, trying desperately to not do or say anything which might "set them off". It seldom works, but you spend 24/7 trying.)
Personally, I think Beinash was totally overwhelmed.......six kids, abusive husband, overcrowded living conditions, having to conceal the injuries to Sara and the fact that her brother-in-law was living with them, allegedly estranged from her own family, etc etc.
Don't get me wrong, I'm in no way defending her, but I wonder if she did - at one point, at least - love Sara, and if the photo of a made up, happy looking, Sara was done out of affection, rather than anything sinister. Possibly to try and compensate for the abuse Sara suffered at the hands of her father.
She said in a message to her sister (in a previous post upthread) that Sara reminded her of herself when younger.....that suggests to me that she cared for Sara on some level - early on, at least.