According to one press report he did allow others to use his computer so it may not have been him who downloaded it.
I can kind of relate to that, some while ago I had someone who would hitch hike down from Aberdeen when HE felt like it and would crash at my place whether or not I felt like it.
He was last down in 2001 before my current computer.
Any hue when I went to bed he would have late night session looking at what I would call pornography, but he said was erotica. (IIRC he says you can not define pornography) It was his surfing one time that placed programs on my machine that would want to hijack my dialup to premium lines.
I even had someone use my credit card fraudulently, possibly because one of the sites he visited planted spyware on my machine which stole my details on logging in to my internet banking.
But also, one time I took a regular back up I had made of my own photographs on a CD ROM, put them in my father's machine and found a folder that I had not known about.
Apparently my father had got windows explorer set to reveal hidden files and folders, given his self admitted lack of tech skills not a good idea.
When I investigated this strange folder, I discovered that my unwelcome house guest had downloaded pornography,
(Sorry no such thing as pornography it called erotica), put it in a folder and made it a hidden folder.
I was luck my father was not watching over me when I discovered it
So getting back to the story in hand, I can personally see that it is possibly that while the images were on his machine, the stepfather may not have had anything to do with them.