Frankie Smith is said to have developed a “strange and unhealthy obsession” with her new girlfriend and nightlclub security worker Savanah Brockhill at the expense of her daughter, Star Hobson.
Smith, 20, and Brockhill, 28, stand trial charged with murder and causing or allowing the death of the tiny tot in their flat in Keighley, on September 22 last year.
Alicia Szepler, Smith's sister, today told Bradford Crown Court that “everything changed” when the couple started dating.
She said a “disconnect” emerged between Miss Smith and her baby daughter and the rest of the family.
Miss Szepler agreed when Zafar Ali QC, representing Smith, said: “They'd sit on Facetime for hours and be texting all the time, and Smith would have to be prompted by you and your mum to look after Star."
The court heard how the relationship became violent as it progressed and miss Szepler saw an incident at The Sun pub in Bradford where Brockhill punched Smith in the face.
Initially, Brockhill tried to brush the assault off as a mistake, but Miss Szepler confirmed in her police interview that there was intent in her actions.
In her statement, read out in court by Mr Ali, Miss Szepler said: “Savannah tried saying she didn't even notice Frankie was stood there, and she'd done it by accident, but I literally watched her face and watched her do it, she literally just punched her in the face."
Mr Ali asked Miss Szepler: “Two days later, when you next saw Smith, she had black eyes and bruising on her legs and arms." Miss Szepler replied: “yes”.
In a text exchange read out by Mr Ali in court, Brockhill accused Miss Szepler of being a liar in reference to her claims, telling her: “I've had enough of what's going on and accusations from last week."
But in response, Miss Szepler wrote: “Where have the bruises come from? I know Frankie is a liar, but you are both as bad as each other.”
The court heard of another incident when Miss Szepler was babysitting her niece, along with her aunt, Antonia, who put a picture of the baby on Facebook after they went to get “chocolate”.
Mr Ali said: “Smith got very upset when she saw the photo and begged for them to be removed, as she was scared how Brockhill would react to the photos."
Miss Szepler agreed.
The court previously heard how Star died after the veins carrying blood back to her heart from her legs and organs of the abdomen had been torn, causing major blood loss.
Prosecutors say the injuries were caused by a severe and forceful blow or blows, “either in the form of punching, stamping or kicking to the abdomen”.
When paramedics arrived at the flat, they found Star apparently lifeless, pale and wearing only a disposable nappy.
She was in cardiac arrest and as they attempted CPR, Star vomited 'large amounts' of brown material and a suction device was used to remove it.
She was then rushed to Airedale Hospital where clinicians did everything they could to save her but she was pronounced dead that afternoon.
The trial continues.