It's not exactly uncommon. I doubt it sees much use other than issuing statements like this one.
Whether it's rape likely depends on a proper evaluation of Barb's mental state. Painful sex doesn't necessarily mean rape, especially for an 80 year old woman like Barb who's china probably doesn't lubricate properly. No, Barb's blank ass stares on camera are not actual proof of dementia or a similar condition. It could be completely consensual incestuous sex given they're history of inappropriate conduct. Barb's mental capacity and testimony, a proper confession from Chris (to police or in court), and evidence of rape will all be deciding factors. If Barb is too far gone to consent she's likely too far gone to give reliable testimony as well. All that is actually known is that Chris admitted through texts to an uninvolved third party (at the time, assuming that they weren't coercing Chris into relations with Barb), which isn't enough to convict Chris, it's really only grounds to investigate. This text confession likely won't hold up in court unless the the phone carrier or app staff is subpoena'd and hands over chat logs, and even then that's still not concrete proof.