As I see it if I were sitting on a jury hearing this case it comes down to these questions.
1. Was Barbra mentally capable of consent/aware of what was happing?
2. If yes to one did she consent? ( as horrible as that thought is)
3. Is Chris fully capable of discerning that what he did was wrong?
4. To what level is Chris capable of understanding consent and incest?
If the answer to one is no then Chris goes away, whether he goes to prison or the state fruit and nut factory really depends on the answers to questions three and four. I really don't want to think about what yes to one and two would mean, that would be beyond fucked up, but I don't think I could in good conscious put either of them away in that case, fines, community service for Chris, and separate living arrangements. In that case they would be freaks, but adult consenting freaks. If he forced himself on her in any way he needs to go away for as long as possible but without a report from a psychologist I don't know where I would send him.