Incest would be easy to prove, but rape gets trickier. There’s only one real witness: Barb herself. Can you see her taking the stand against Chris? Even if she was lucid enough to testify, would she? I doubt it.
Also, given her mental state, her literally asking Chris for sex and/or isn’t out of the question. And that’s where Chris being a retard comes into play because he doesn’t understand how dementia works and what their confused state can lead them to do. Chris may have honestly thought that Barb was a willing participant.