I'm afraid that just because they said it could be done is not actual proof that it could be done. It should have been checked that it could be done by the police which I believe was not done. So the onus is on the family to prove it. Not upon the defence to prove that it cannot be done.
The prosecution was under no obligation to prove how the entry and exit occurred. It is not an element of the crime. They offered several pieces of evidence that it could be done though they didn't have to. The fact Sheila didn't kill herself was enough to say it happened there was no need to prove how entry occurred. Only the element of a crime must be proven beyond a reasonable doubt.
The defense in this case did need to rebut the claims made by proving it impossible. But they couldn't indeed a ladder could have been used even. That is why the appeal court declared:
"The Crown did not have the burden of proving by which window and by which mechanism the entry was made. The Crown proved capacity both to enter and leave."
At any rate he stated he could get in through the kitchen window:
"He told the police that there were occasions when he gained entry to his parents' home by way of a number of the downstairs windows including those in the kitchen and the bathroom. He explained that he used a knife to move the catches in order that the window could be opened from the outside."
http://www.bailii.org/ew/cases/EWCA/Crim/2002/2912.html