I sadly haven't been able to reproduce your exact problem. While it's possible to force an error shortly before the end of the 'evil' scene – by using the Shaddy/Witcher-mod but neglecting to properly extract the folder 'shaddyMod' into the game's 'game' folder – said error can simply be
Ignore
d and the game will continue as normal:
If the above doesn't help in solving your problem you should remove all mods but the incest one (it's just a bunch of definitions, it won't break anything) and revisit said scene to see if the problem still persists. Do take note that your saves probably won't work with the official script files, but as the scene marks the beginning of v0.3 you can simply skip to it after starting a new game.
In case the above mod is to blame you should probably ask the modders for further assistance (
here). Or, if your patience has run thin, switch over to
scrappy's mod or the official script files and in case of the latter simply use an editor to inspect the outcome of the choices or have
0x52's
Universal Ren'Py Mod do it for you.
Hope at least some of it helps!