Correct. One can only plead the fifth if the information requested could lead to a criminal prosecution, which would typically not be the case in a divorce or child custody case.
Let’s play a little “what if” here.
What if there is a criminal matter still hanging in the breeze that isn’t being discussed in the civil court? What if she is the only person who even knows the criminal case is out there because the authorities haven’t yet been informed of it? What if she is concerned that answers she might give in the civil case could be used in a criminal case later?
There is a film-clip making its way around the net which shows a law professor in Virginia telling his class of budding attorneys they should instruct their clients to NEVER EVER talk to the police. EVER!! That clip goes right to another in which a police detective says the same thing!
The simple fact is the authorities have the right to investigate anything, anytime... actually, anyone can investigate anything, anytime so long as they follow certain rules. But you have the right - per Amendment 5 of the U.S. Constitution - to NOT help. That doesn’t mean you can hinder the investigation but you don’t have to help. The same rules hold in court. You don’t have to help the prosecution, the plaintiff, the state or even the old lady who falls down in the aisle.
“... nor shall any person... be compelled in any criminal case to be a witness against himself...” If she knows of a possible criminal case that could be brought against her and is fearful that her answers in the civil matter might tend to incriminate her later... she should invoke the 5th until she gains blanket amnesty from the court.