Well, I would be interested in knowing the respective ages of everyone involved, but I do have a few ideas, off the top of my head:
A solo game is pretty easy, but instead of sticking with one New God as an NPC, simply develop a few NPCs who are recurring characters - maybe they're part of the same pantheon, are gods the character can go to for advice, etc.
Secondly, it's always fun to figure out what your players actually like. For example, with my close knit RPG group (consisting of my wife, brother and sister-in-law), the game would be quite different than something involving, say, my daughter.
With your little sister, it might be more rewarding (even for you) to give her a chance, but involving themes that SHE will enjoy. For instance, I have a little D&D game I run for my daughter (and wife), but it focuses on such things as friendships, animals, and the beauty of nature. Of course, there are also villains and such, but they are fitting and appropriate to the setting or theme. Any fantasy-themed Disney or Barbie (like Princess and the Pauper or Mermadia) provides plenty of amazing examples.
Naturally, with more information, I could provide more ideas, but I think you have the necessary tools to get something fun started - you've already got a brother and a sister who are willing to play, so I think you could make the best of it.