Legally there is a difference between incompetency and insanity, but if they deem Barb no longer has the mental capacity to truly consent, they will charge regardless.
However, I was just reminded of this gem from Massachusetts last year. Wife walks in on her husband (40s) fucking his mother (60s), apparently consensually, after a round of vidya. Wife rightfully shat herself and called the cops. They were charged regardless.
The wife who found her husband having sex with his mother on their couch says she always suspected something between them and reveals the very similar way their own romance began.
www.news.com.au