Only after I'd been married to my husband for some years did some of their mutual friends admit that they'd witnessed his ex's abusive behaviour. Her narrative had been that DH had run off with a younger woman and that her boyfriend was merely a work colleague with whom she'd gradually become close.
In actual fact, she'd become close with a younger work colleague, culminating in the pair of them sharing a room during a work team-bonding trip.
My husband's adult children had no idea since they were grown and away from home when it happened.
In public, the ex had sniped at DH at any opportunity. This had been passed off as his ex's 'sense of humour'.
Interestingly to me, the only people who knew the truth were those who'd known both my DH and his ex since childhood - apparently she even had form for sniping at people when she was a pupil.
She was a master at making bitchy remarks in public.
One time, DH and I were invited to a friend's landmark birthday. The venue was the church hall of the village where DH had grown up.
When I walked in the room, to my consternation the ex and her BF were there. [Of possible note: the ex was used seeing me overweight and dressed in dull colours. I'd lost 4 stones and was wearing a really nice outfit for the first time in my adult life.]
My husband moved away from me to speak to an old friend. Just then, the ex pounced on me. Somehow, she'd assembled an audience of village matrons as she intoned: "My, Weary, what a lovely outfit! What a lovely colour! Mind you, you can wear it being so YOUNG!"
I said "Thank you," and she swept away.
I'm sure that those watching would have thought her behaviour fine - there's no way that they would have known that her partner was a younger affair partner or that she'd swindled DH over the finances or that, just after the two year separation period was over, she'd tried to talk my husband into going "back home". (I often wonder whether her boyfriend knew about that.)
My point is that it's quite plausible to me that @GotTheBluePeterBadge 's BIL might have no idea about the ex's true behaviour.