An investigation into the death of Poppi Worthington was "unstructured and disorganised", a leaked report says.
The 13-month-old girl was sexually assaulted by her father before her sudden death in 2012, a judge ruled.
Paul Worthington, 48, was arrested but later told he would face no charges. He denies any wrongdoing.
The Independent Police Complaints Commission (IPCC) report says the investigation into her death was "not conducted diligently and expediently".
A judge criticised Cumbria Police for the way it handled the initial investigation into Poppi's death, with evidence lost.
The report states it took acting Chief Constable Michelle Skeer, who was then assistant chief constable, more than one month to hold a meeting to discuss those criticisms, and when it was held, it lasted 30 minutes and they did not go through the judgement.
The IPCC said this "would not have been time to go through the all the points raised in the judgement".
Paul Worthington's sister Tracy said: "That [the lost evidence] would probably exonerate him, he has been accused of something he hasn't done. How can he clear his name when the evidence has gone?"
'Unstructured and disorganised'
Mrs Skeer is also criticised for instructing another officer to "carry out an internal assessment into the Poppi case rather than refer her concerns to the Professional Standards Department".
The report points out that she had previously been the head of professional standards and should have known a referral was the "most appropriate action".
John Woodcock, Labour MP for Barrow, told the BBC said he wants the police commissioner or the home secretary to look into how Mrs Skeer is temporarily running the force.
"This report is nearly a year old and the decision for the deputy chief constable to step up to be acting head of the whole force, it is really concerning given the scale of the criticism of her personally within this report, which still remains hidden from public view."
The report concludes that "there is substantial evidence available to support the contention that the reason this case has still not reached a resolution… is because of the unstructured and disorganised approach."
In a statement, Cumbria Police said: "In April 2014 ACC Skeer asked the new temporary head of crime to assess the adverse judgement from the family court.
"Following completion a referral was made to the IPCC in conjunction with Professional Standards.
"Mrs Skeer provided a witness statement to the IPCC as part of this investigation.
"In June 2014 Mrs Skeer arranged for a second investigation into Poppi's death."