This muggle-born witch was particularly skilled at potions. She helped the Chosen One on many occasions, even saving his life.
Petunia's Sister is located on the Fred Haught/Arizona Trail 1.7 miles from the southern trailhead. The coordinates are very imprecise due to tree cover and hills (I took 9 readings with a total variation of 75 feet), so you will need to use the description to find the cache (just like a letterbox). Hike the trail until you reach the appropriate waypoint. There is a small canyon to your left (west). Go about 125-150 feet up this canyon, looking for a cluster of large rocks on the south side. When you are standing at the base of these rocks, which is on a steep sidehill, there is a small recess in the rocks about head high that is 3X3 feet. This is the hiding place. Use the spoiler picture! [The terrain rating is based on the last few feet after hiking the trail; it would otherwise be 2.5. If you elect to hike in from the nearby forest road, increase the terrain to 4.5, prepare for a steep climb down and ascent back up, and don't expect to save much time]
This is a letterbox hybrid. In addition to the normal logbook and trade items, it also contains a letterbox stamp, inkpad, and stamp book. Please do not remove the stamp, ink pad, or stamp book. THEY ARE NOT TRADE ITEMS!
There is a clue in the logbook and stampbook that will help you with an upcoming letterbox (particularly the actual stamp). Make note of it.