Savannah Brockhill (28) was found guilty of murdering Star by a jury who heard the child endured months of assaults and psychological harm before suffering "utterly catastrophic" injuries in her home in Keighley, Yorkshire.
Brockhill was the partner of Star's mother, Frankie Smith (20), who cried as she was cleared of murder at Bradford Crown Court yesterday but convicted of causing or allowing the toddler's death.
The verdicts come in the wake of the widespread outcry that followed the case of murdered six-year-old Arthur Labinjo-Hughes, from Solihull, near Birmingham, this month.
Outside court, Star's great-grandfather David Fawcett (61) said: "I'm just pleased we got a murder conviction for Savannah Brockhill.
"To me she was just pure evil. I just can't believe she could do something like that to a baby girl.
"We were just a quiet, lovely family and she ascended from the bowels of hell and just completely devastated and wrecked our family."
Mr Fawcett's partner, Anita Smith (70), was one of five people who reported concerns about Star to social services in the months leading up to her death in September last year.
Asked about the response of social services, postman Mr Fawcett said: "It's disgusting, because there were five referrals. Not one of them did anything. It's just beyond belief."
When asked whether social services had missed the "blindingly obvious", Mr Fawcett, who is the grandfather of Frankie Smith, said: "Yes."
Bradford Council leader Susan Hinchcliffe said a review was under way into agencies' contact with the family, but added: "Star was let down and we all want to know if anything could have been done differently."
Local MP Robbie Moore called on Ms Hinchcliffe and the council's chief executive Kersten England to resign, saying: "Bradford Council bosses should hang their heads in shame."
The verdicts came less than a fortnight after Emma Tustin, the stepmother of six-year-old Arthur Labinjo-Hughes, was found guilty of his murder in a separate case that caused shock and revulsion.
Britain's Department for Education said the death of Star was "deeply disturbing" and said it would "not hesitate" to remove children's services control from Bradford Council "if necessary".
A department spokesperson said the review into Star's death will feed into the national review commissioned last week into Arthur's death.
Smith cried uncontrollably as the verdicts were delivered while the pair stood in the glass-fronted dock.
Star was taken to hospital from the flat where she lived with Smith in Keighley, but her injuries were "utterly catastrophic" and "unsurvivable", the prosecution told the two-month trial.
Jurors heard Smith's family and friends had growing fears about bruising they saw on the child in the months before she died and made a series of complaints to social services.
In each case, Brockhill and Smith managed to convince social workers the marks were accidental or that the complaints were made maliciously by people who did not like their relationship, the court heard.
Prosecutors described how the injuries that caused Star's death involved extensive damage to her abdominal cavity "caused by a severe and forceful blow or blows, either in the form of punching, stamping or kicking to the abdomen".
Jurors also heard there were other injuries on her body which meant that "in the course of her short life, Star had suffered a number of significant injuries at different times".
Brockhill and Smith, both from Keighley, are due to be sentenced today.