normal family who have done their best, and ended up with one (female) child who had turned out lovely, which highlights that they were not the cause of the boy’s behaviour.
I've been thinking more about this and the comparison you made between the Jamie and his sister in the context of parenting
I think a really important comparison is between Jamie's dad versus the male DCI. And the DCI's son versus Jamie
Both dad's have traditionally male jobs, both are physically strong manly men.
DCI is self reflective. He says he's a 'soft touch' and leaves his wife to do the discipline in terms of not letting his son miss school. DCI has maternal aspects to his parenting (his response might be "aww poor baby, don't go to school if you have a tummy ache" and he knows that it's not helpful.) His wife has paternal aspects in her parenting ("you're going to school" and their son obeys). DCI is taking his wife's advice to eat apples when he wants a smoke to help him quit. Would we expect Jamie's dad to take his wife's advice?
Both dads seem to be out of touch with their sons
Neither son is strong, well built, 'masculine'. Both are bullied and lack friends.
DCIs son seems like a good kid - no anger boiling just under the surface, helps his dad understand emoji codes. He's aware of, but not radicalised by incel narratives.
Both dads see what's going/gone wrong at the end of their story arc. DCI finds a way to connect with his son over the best chips in the area and ribbing about his terrible french. There's hope there
Jamie's dad left it too late