I believe both Batool and Urfan abused Sara, and Batool just pretended to her sister that it was solely Urfan.
I wonder if Batool sent those messages to her sister as a way of "showing off" about the abuse she had carried out, under the cover of pretending to be concerned for Sara. I also think she sent those messages as a way of providing a cover story that she was innocent, in case anything happened to Sara that they couldn't cover up.
If she really was concerned about Sara, she would have called the police/social services, and she wouldn't have gone on to have four kids with Urfan.
The fact that she refuses to provide dental imprints to see if they matched the bite marks found on Sara's body speaks volumes. There's also testimony from neighbours saying they heard her frequently screaming abuse and swearing at her children.