Thank you so much everyone for your advice so far. It has all been very helpful / affirmed our own thoughts.
My sister has a solicitor now, every email/phonecall is soooo expensive and she is slow very to reply. We are desperate for any help/advice.
Some things he has done since I posted last:
Gone to my brother's workplace and made serious false allegations against him. No one believes them but procedures must be followed and it has caused much upset for our brother and us. We weren't expecting something like that at all.
Racked up debts in my sister's name on two credit cards that he had taken out in her name. One is old (2018) and she was aware he had it but never knew it was in her name - she never had a card only he did. He opened it without her permission. The second one she had forgotten he had taken out in her name whilst they were together (citing some reason or other why it had to be in her name). She has frozen both now but is being told by the respective banks she must pay. She has also recieved a tax bill from HMRC - evidently he had been paying her from the company he works for to cheat tax. Of course, she never saw any of this money. She owes thousands altogether.
He has stopped getting paid at all - though we know he's working and a well-paid job -just being paid indirectly. Child maintenance has been calculated at 40pounds a month.
He is staying in the house. She wants to do an occupation order but wants to get her things first (as she is sure he'll destroy them otherwise) but he won't allow that (doors locked from the inside - and he's got a dog now - aggressive breed). He claims the police say he's not allowed to give her access to the house. She is waiting for a police escort to get her things. We have been told if she doesn't get an occupation order then it will weaken her position of needing a home for her and the girls in the eyes of the court. At the same time, the police say they can't force him out if he doesn't vacate willingly (he won't).
He has stopped paying the mortgage on the house and holiday let. The mortgage used to go out of her account whilst they were together (I know!). She stopped the direct debit and he somehow re-instated it. The bank reimbursed her. Regarding the holiday let - he claims it is empty - but we know he has long-term renters in there - again he must be getting cash in hand. Whilst they were together he had told her that it barely covered the mortgage. We now know these are lies and it makes a lot of money each month.
For the sake of the girls, she has been allowing him to spend a few hours with them every weekend supervised by a trusted mutual friend with the okay of their support worker. However, the last time they came home distressed because he keeps speaking to them about his separation from their mother. Her solicitor has advised contact should only be every other weekend and through the contact centre. She just wants to do right by her girls. We have heard that he has been threatening to call social services and tell them she is a bad mother. She took anti-depressants for a couple of months a few years back. Apparently, this is proof that she is a bad mother.
He cannot go near them at the moment (bail condition) but his bail will end soon. Our understanding is that she can then apply for a non-molestation order but that it will take 24 hours and will leave her / my parents unprotected for those 24 hours. We have been told this by several different professionals but surely this can't be correct?
Thank you if you managed to get this far. We feel like we're living on a knife-edge just anticipating his next move. My sister is in between jobs with no money in her account and debts that he has racked up in her name. I've never personally met anyone as toxic or as sly as him, but I think as a previous poster said he's not as clever as he thinks and it will all come down on his head in the end. Any advice much, much appreciated!!!