They would arrest her for Contempt of Court. The Courts are the law of the Land and people must abide by their rulings. She could be fined and/or given a few months in jail. This process would just go on and on until she abides by the Law.
The court case concerned might be relying on your wife's evidence to get a conviction.If they have a lot of other witnesses to whatever crime was commited they might not need her to testify.I would definitely contact the police and state that you are concerned for your family's safety if she does testify.
But, of course, as the old saying goes, "You can take a horse to water....."
Many "reluctant" witnesses are summonsed to give evidence. This is particularly so in cases of domestic violence. It is amazing how many of them "forget" all the details of the incident(s) they are being questioned about.
So long as you turn up, you cannot be prosecuted for having a bad memory.
It depends on the case and what she was a witness to. The prosecutor could ask for a witness warrant and your wife could be arrested and brought to court.
If she refuses to give evidence and she has made a statement in the proceedings the prosecutor can apply to have the statement put to the court as her evidence in chief under the hearsay provisions.
This is very useful in domestic violence cases when defendants think they can get away with it by persuading their partner not to come to court