Again, NOT normal. This isn't forgetting where your hairbrush is, or occasionally neglecting something on the stovetop and having it boil over; any well-adjusted adult would take precautions to prevent losing such important and costly items.
It's only explainable if he has some type of organic brain disease or injury. Is there a history of early onset dementia in his family? Has he ever had a head injury; recently, or in the past?
I think he's having a type of emotional breakdown.
Yeah, it's not possible for him to brush this off or forget it; though, of course he will say that he can. "it's fine, it's in the past, it's over" is classic denialspeak.
Denial is good to an extent. Its function is to allow you to survive the horror of a situation in which you are powerless.
But it's only supposed to get you through for a short period of time. Eventually, sooner is better than later, you have to admit to yourself that what happened, happened; and that you are dam.n. furious about it.
Having his dad fink on him is tragic, just awful. You know of course, that at a deeper, subconscious level; your husband thinks that he is guilty of something that caused the molestation. All victims of abuse think this. But, if a loving parent is able to validate them, and advocate on their behalf----what an aid to healing and recovery.
Being abandoned by your parents in such a situation will cause additional, profound feelings of worthlessness.
You may have to threaten divorce. How long can you be married to someone who, really, in a sense, is losing his marbles? You can't be both adults in the relationship.
If you threaten divorce; you have to mean it though. No bluffing.
Have you told him that you think he is in denial about what his stepbrother did to him? That this will never, ever "go away" [if only it would] Before divorce, you should have a very frank, gut-level discussion with him about this.
I'm guessing he's having some real issues with his sexuality. The psyche cannot bear what your husband has had to carry. At some point, a breakdown is inevitable if the pain, and it's cause are not remembered and confronted.
The more I type, the worse I feel for him. But I feel just as bad for you. You are carrying way too much responsibility for him; and going without sexual affection.
I get the feeling that you are in Britain? Does this group sound helpful? https://www.havoca.org/