How do you trigger the random event that allows you to get the Obscuridad del Santo Andres? I am already at Hero fame status and have wandered around Las Hermanas for some time.
Apparently it varies widely when you can receive the item. I had max honor before I even opened up Mexico. I received the rosary in Armadillo when I was heading out of town at one point. Funny story, I actually accidentally ran her over with my horse the first time because I didn't see her, took a hit of -100 honor and had to reload my save. :P Note, you do not need Max Fame to get this, as I received mine before my Fame was at max.
Popular places to check:
There's a lot of conflicting information out there, unfortunately. Some people have received before donating to the nuns, some people never donated to them and received it (like me), the locations and time of day you see her are also random.
Have you done lots of side missions/random encounters yet? When speaking with her, she says that she's heard you've helped lots of people, so that is suspected to play a factor. Good luck!