Again I am sorry - we are all working lawyers with ordinary lives and I consider it too late at this time to call you.
However I will happily call you in the morning
The way forward is for your sister to find a family law specialist using the website here
She needs to find a solicitor she can trust and work with - and if that means visiting two or three to start with then that is not unreasonable - she must feel confident in the relationship.
She may also wish to consider using a Collaborative lawyer
Although from what you have said this would not be appropriate as it is unlikely that her ex will be reasonable
For the same reason Family Mediation
is not a viable option - although it is possible that she will need to attend a Mediation Information and Assessment meeting IF a financial applictaion needs to be made.
If it is safe for her to remain in the matrimonial home then she can start divorce proceedings based on her husband's Unreasonable Behaviour
You can read more about the divorce process here
However if he has or does make threats against her then she can apply for an Order allowing her to remain in the Matrimonial Home and Excluding him.