The older you get, the more cognitive decline you get, you can fill it in with experience, and fake it. There's no evidence Barb is actually demented, though, are you people fucking retarded? Just because someone is old doesn't mean they're automatically senile. Barb has done shit like talking like she's senile on a video but then a day later she's entirely coherent when yelling at Chris.
It's absolute bullshit for Chris to end up getting the complete blame for whatever goes down unless Barb is full Alzheimer's. It's just as likely she was the one raping her own retarded son.