Bastard! Alongside serial killers these gits are in my opinion among the best psychological profilers in the world - they know exactly which buttons to push.
Encourage your sister in as positive a way as possible to get therapy, to get support.
Keep boosting her self esteem. Tell her all her positives as often as possible, highlight the positives of her life away from him, of how happy their dd is when it's just her and dd. Get her involved in her local community if possible. Keep her busy. Give her things to look forward to - help her create a vision of a happy future without him (though I'd avoid the idea of another relationship).
I do hope you are able to help her stay away from him and build a future for her and her dd.
It's an extremely complex dynamic which I don't think people who haven't experienced it understand.
I'm the child of an abusive home, my parents are still together. When I was younger I just wanted mum to leave. Now I'm older I can see how he's managed to make her truly believe she could not cope without him - even though the realty is its him who wouldn't cope (she's his carer he's basically bedridden now)
They've been together over 40 years all her adult life basically. She'll never leave.
I've read a lot on it and there are certain experts who liken it to "Stockholm syndrome" and the brainwashing those in cults are subjected to.
I remember watching a show years ago and the host played a "trick" on the audience - planted a researcher who loudly complained about a bad smell and convinced the queuing audience, well some but a good number, to also think they were smelling it. Then when they were seated "apologised" for the issue blaming something like a blocked drain iirc. Then when the show started admitted it was a ruse - with a point.
They were only queuing for about an hour, had no emotional relationship with the researcher and yet they'd been convinced.
Host then said something like 'now imagine you've someone you care about even love, telling you every day for years that you are ugly, stupid, weak, couldn't live without them etc. Can you now begin to understand why victims of abuse find it so hard to leave their abuser' and the programme continued with guests who'd been victims of abuse expanding on this theme.
The mental abuse is far harder to untangle than the physical.