The way I understand your question is: You gave an earlier statement to the police, your fiancee was  charged as a result. Now, the court is trying to see whether the charges against your fiancee are proven beyond a reasonable doubt, and to do that you may be called to testify.

The problem you are facing is that if you are called to testify truthfully, and do not, will the prosecutor decide to charge you with perjury (which is the technical name for making a false statement)? Perhaps you should seek a one-to-one consultation with an attorney to see what else you need to defend against.

"Pleading the fifth" is not only available to defendants, but also to witnesses, who may be chargeable as defendants.

Again, I suggest you talk to an attorney (preferrably not the same attorney your fiancee has) for more advice.