Ok so that's just one too many coincidences isn't it?
So you were abused as a child and your mother made it crystal clear that her choice was, at best, to fail to support you: at worst, to then knowingly continue to leave you in harm's way.
Now you have your own child and your experiences have made you vigilant - surprisingly however, the one person you will leave your child in the care of is the one person who has demonstrated how skewed her boundaries are on this and how little she is to be trusted.
And lo... the very first (I think you've said it's actually the first) time you allow a sleepover situation, something like this happens involving the man your boundary-skewed mother has chosen to be with. Right.
Either this is a troll thread (apologies, I say this simply to underline how eyebrow-raising this situation is: I have no reason not to believe your story) or there are one too many coincidences here and I would have a real problem passing this off as some sort of mistake.
Your mother thinks child abuse is no big deal.
Your stepfather, her choice of man, leaves porn open on his phone and hands it over to an 8 year old by mistake
Your mother chooses not to tell you that your child was exposed to her husband's porn and shrugs it off.
You need to draw a big fucking red line here and start asking some hard questions on whether it is a good idea to have this woman (and her choice of man) in your life.