In order to convict her, they have to gather evidence to support their allegations, Jaycee simply being on the property wouldn't be enough. If PG, Nancy and Jaycee all remained silent in court the evidence would be limited to Jaycee being on the property (which is not in itself a crime since she was an adult at the time) and the DNA of the girls. Since the girls have no birth certificates recorded when they were born, their ages cannot be proven with a high degree of certainty, which means that in a worst case scenario the only thing that can be proven with certainty is stat rape charge in the case of Angel's conception. So, testimony from at least one of the three is crucial to the states case in general. Knowing that someone did something and proving it are two different things.
From all accounts Jaycee is co-operating with the DA's office, so there is no reason to think she won't testify. But, in the event she would not or could not, the state would need testimony from one of the other two or their case is going to have real issues. And if she testified for the defense, that would be devastating to the prosecution (very unlikely that would happen though).
PG is obviously the bad guy, so there is no way he is getting a deal. He is going to prison for the rest of his life no matter what the outcome of the case is. If he testifies it will be an act of contrition associated with a guilty plea. I don't think there is much chance of that - could happen, but I doubt it.
That leaves Nancy. Again, she could plead guilty and testify as an act of contrition, but it is more likely that she would be offered some sort of deal that would offer significant jail time but still allow her to get out before she dies. That would pretty much seal up the case for the prosecution and would provide the public with a much clearer insight into the facts of the case than would otherwise be possible.
That would be dependent on the role that Nancy played in the last 18 years of course, if she was the dark force then that would make things complicated. On the other hand if she is a more sympathetic figure then they might cut her a break - there is nothing to be gained from subjecting her to extreme punishment. And her role would be clear to the prosecution based on interviews with Jaycee and the two girls, so they wouldn't be going into that blind Karla-style.
The important thing here IMO is to make a public example of the source of the evil (PG). I think showing mercy to peripheral characters sends a different kind of message, one to similar sorts of characters out in the real world right now, that if they turn, co-operate and/or end the situation they are in, that they are not going to be punished to the max. Otherwise they have all the incentive in the world to keep quite, and that is not in society's best interests.
