Feel a bit sad for the old man. Had a feeling that was the twist.
Also, stepmother? Did we know she was Yuuma's stepmother and not his biological mother? Where IS his biological mother?
It has been hinted at before. Back in Ch. 39 she mentioned nothing good ever comes from being
involved with him, which would be a particularly estranged way to refer to their relationship if he was biologically her's. Also in Ch. 66 after phoning the police she mentioned always being creeped out by Yuuma's presence for as long as she's known him and was glad to be rid of him from the house. It's important to note she's still wearing the bandage dressing from having her hand impaled back in Ch. 20 and the neck brace from the injuries she suffered in the Umakin attack back in Ch. 30.
As far as the timeline, Yuuma mentions his Dad comes home on Fridays (shown to be particularly violent at those times, perhaps drunk?) such as in Ch. 36. Every chapter is a different day which Yuuma marks off on his calendar on the wall in the background of his room. So therefore he tends to show up after every several 7-chapter incremental multiples thereof like in Ch. 57 and again later in Ch. 85 when he got ceiling Hina'd.
Yuuma also stopped going to school in Ch. 48 (or roughly one and half months ago now in-universe) following the death of the first police officer and the ensuing investigation that his Dad and Stepmother were suspiciously hostile toward. Also interesting to note Yuuma does have a modest recurrent wardrobe, with the dark star t-shirt he wore this chapter also seen previously like in Ch. 40.
.....When the AI is more of a human than the actual person. Don't I know it.
Interestingly enough this wasn't even the first story to deal with the concept of AI, which was central to Ch. 71 along with the same theme of humanification.