I can’t understand the comparison with Diablo since I don’t play it, but I played Ori and I know for sure that Ori is easier than NRFTW.
NRFTW’s combat system is inextricably linked to timing and coordination, and the only way this game can be faced by someone with your sister’s difficulties is to trivialize it through a specific builds (since your sister isn’t good at building, you could help her) or a customizable “accessibility mode” instead of the currently implemented easy mode, which is something I’ve discussed in this topic: Game Accessibility and Difficulty, Unsolved and New Problems (Breach Refind).
Multiplayer will be added soon, and perhaps you could help her directly in the game by luring enemies, making combat easier for her (but you may have to buy two copies of the game 
).
This thing you are asking for is something that requires a rather high amount of time and resources to be implemented well, and I find it unlikely that it will be implemented, At the moment the developers are struggling to even make the “normal” tutorials perfectly understandable, and many people complain about how the game doesn’t explain some of the game mechanics (although they are making progress on this front).
Realistically I think the most helpful thing the DEVs should do to help your sister play the game is the “Customizable Accessibility Mode” I mentioned earlier and as for the rest like builds and places to go you’ll have to help her if she can’t figure it out on her own.