Ok unpopular opinion time.
All elder people have memory problems. Doesn't mean they can't consent.
But in any case, even if she had dementia , if shes literally asking for it, what harm does it do? I really don't understand why people try to paint it as rape. People can think t's gross because of the age and because the incest and that's probably why they don't care that there are still laws that ban two consenting adults from having sex but doesn't change the fact that this is literally a victimless crime.