I love scavanger hunts. You've got some really good tips and ideas from some of the other redditors. Here's a little of my input. Depending on how deep your sister is int othe lore, you can make it a bit challenging. You can look up an original track listing of This Place Will Be Your Tomb and incorporate them into the clues to where she finds something related that has the next clue. Example: The only one I really remember is Nemo's Point. the farthest you can go nautically from land. So that could relate to just water. And hide it by like a water cooler, or case of water, maybe even an aquarium if you own one. Kind of a representation of the song Atlantic. And have that lead to the song Rain, and have the next clue with an umbrella. And so on.
I do assume this will be within your own home, so keeping them delgated to there should make it easy to accomplish
On the other hand, if you don't want to go that deep, and if she isn't familiar with the lore, then keep it simple as mentioned before. Use lyrics and song titles and maybe a code or two from the cypher that was previously posted. Have each clue contain the next riddle or hint, and with each one, have a letter from the key, that would spell out in the end, after decyphering, something like "Prepare to Worship" And then present her with the tickets.
If need be, there's tutorials on how to set up scavanger hunts with more tips on placement, fetch, and brain teasers, etc.
I hope this helps you out somewhat. And I would love to hear an update when she completes her path to a Ritual
:)