Fly 
Please take it from me, you are a great sister. I have a great sister and a great mother, and great relatives. I know one when I see one in action.
Here is what to watch out for now:
Gestures of sorrow and remorse on the part of the STBX.
Expect flowers, professions of love, apologies and promises of massive change. Do not let your sister fall victim to any of this. It is all fake. There will be cycles of 'remorse' and anger on his part. He will be very angry that he is not dictating the terms and very anxious to reel your Dsis back in.
A certain amount of revision of history on the part of your Dsis.
She is no doubt feeling terrified right now, frightened of what this man will do to her in court during divorce, frightened of having her whole life laid open for all her friends to see, feeling ashamed and humiliated and small and powerless and frightened of the judgement of others on her (Why did she stay? She must have stayed for the money... She must be making it all up, he's a doctor after all... Why did she not leave for the sake of her daughter if it was that bad?) It is very natural to start asking 'Was it all really that bad?' - 'Could we not work on it - he seems to have his tail between his legs and it might just work..' - 'He once said he loved me and he was nice to me two Christmases ago'. It is also very natural to try not to antagonise someone like this man by taking the initiative and filing for divorce, or calling the police to report abuse. In order to find an excuse for not doing this, history can sometimes be rewritten. Be kind and patient and bite your tongue when you feel like blurting out something angry. Listen, listen, listen.
Remember she is going through a lot of grief
Right now her life has fallen down around her ankles and she is feeling wretched, a failure, a public failure, which is worse. All of her hopes and dreams are now officially dashed. She has probably known this for years but the toothpaste is now out of the tube and there is no putting it back. There was a time when she really liked this man. Again, bite your tongue and make it your business to listen.
To Do:
If she does not contact Women's Aid by Sunday, I really think you should make that call yourself on her behalf. Women's Aid may also be able to recommend someone for the DN, or they may provide services themselves. This can be your excuse for calling.
She may hesitate to call because it is really, really hard to imagine yourself putting into words something that you have kept secret for so long. She is not at all unusual among abused women in the keeping of this secret (so incidentally, don't beat yourself up about not knowing). It is part and parcel of the effect of abuse that victims keep it secret and find it hard to contemplate talking about it. It is immensely humiliating to be an adult human being admitting that you have been treated so badly for years.
Buy her ASAP the Lundy Bancroft book, 'Why Does He Do That? Inside the Minds of Angry and Controlling Men'.
I really think that you, DB and Dsis and DN should go to the police station and record what happened. I know Dsis may be very hesitant. Maybe after speaking to Women's Aid she may feel a bit stronger.
Please start to scout solicitors with reputations for understanding abusers and domestic abuse situations, and also for being really good on their feet in court.
A word of caution:
You must not record phone calls. It is illegal to do so without the permission of the person on the other end.
You can take notes and send a memo to him or to someone like his mother, stating the contents of any conversation 'for the record' if abusive phone calls take place. You are fully within your rights to tell people on 'his side' to communicate with Dsis and DN only by e-mail and set up a dedicated e-mail address for this purpose. That way they know that what they say will be on record and it may serve to civilise them. This may be enough to put a stop to their gallop.
[I believe it is ok to put someone on speaker, but if any other poster has any concrete info on this, please post.]