I posted upthread about my awful ex who was very like JA in many ways, and from that I think I can guess what the family were thinking.
In my ex's family, my ex was always throwing huge tantrums and threatening suicide whenever he was met with the slightest bit of criticism or not getting his own way. I recognised that these were mostly empty threats, but of course his mother was really worried that he would really do it, so over the years she adapted to accepting anything that kept him happy. Even telling me to be quiet and put up with it while he abused me. Even when I told her he was a serial cheater, she just said that it was his depression and begged me not to leave him. She was just terrified of upsetting him, because his tantrums were terrifying, and she was terrified he would kill himself over the smallest thing. So she just lived inside the lies. She knew factually that he was abusive, but she walked and talked like he was the victim, because that's the fiction she needed to keep him steady.
When people ask why none of Arday's family stopped him from writing this book full of lies - or accuse his wife of coming up with it all - I see this dynamic where behind closed doors, Jason was throwing tantrums and threatening suicide, so the family ended up doing whatever it took to avoid sending him off the rails.
Edit: I don't care when Diane Abbott and the like say he was the nicest person they ever met. Most narcissists are beloved by everyone outside their own families.
The same with my ex and his ex wife, who I was also friends with (all very amicable, or so it seemed).
He knew she was abusive, to him and to their kids, had stolen from her own children, was a serial adulterer, she'd abandoned them and had a new family with someone else, kidnapped the youngest child, had accused him of all sorts of horrendous things and that basically everything that came out of her mouth was a lie.
The architecture of his entire life was centred around placating her because he was terrified she'd kill herself and he'd be the one to blame, as he was blamed for the abuse and infidelity and for her leaving the kids with him. So rather than set a boundary, he basically colluded with her to keep her happy because it was the only way he could keep her from going off the deep end. She said jump, he said how high. I spent more time than I'd care to admit running round in the middle of the night to prevent another "suicide attempt" because they're always the victim and you're always the perpetrator.
And when she died, prematurely and tragically, after destroying every friendship and relationship she ever had, everyone did what they're doing here, which is flatter their own egos by pontoficating on what a great person she was despite all evidence to the contrary, including her poor kids who'd hardly seen her for years.
It is a mind altering dynamic to be part of but it's very hard to respect anyone who goes along with it (hence why this man is my ex). That said, the price of calling them out is often very high. The only way to win with this type of individual is to not participate at all.