Why? You're not forced to include family in your will you know? Especially after so long. Sure, contest it if your dumb and rich enough....probably won't get anywhere though.
So yes, unfortunately, whatever anyone knows, thinks, believes, the game is troped up to hell for that scenario which apparently, was never the case originally.
Different countries have very different inheritance laws, and the will's weight may not matter as much. Cheeky is in the UK, if i recall, so we'll assume he is using his country as the baseline.
I also left out things such as alimony/child support and such, which varies state by state/country.
I know that in certain US states, as monica never remarried, dad would have been writing a hefty alimony check every month if he was this baller that he liked to brag about.
Again, dont know anything about UK divorce laws, so meh.
Either way, if i read the end of your post correctly, we are in agreement that one would have to take extreme liberties with the plot/setting just to include a fetish that wasnt intended.
(To be clear, have no problem with incest. It's just not needed here.)