Another vote for Time Keepers. The idea has all kinds of potential, and it's especially cool if your sister can help you take this from idea to reality while the community pitches in!
i'm intrigued with the idea of the Time Keepers story. For whatever reason, i feel drawn to this story more so than the other one. i could see this one being a fun and thrilling read
I think it would be fun to see how far you can go with the story you and your sister have come up with together. Double the creativity and insight into the possibilities.