You ransack the attic for guidance.

With Jessica out, now is the best time to see if anything can be done about the situation. The attic is a good place to start, seeing as it's where you found the amulet... and you chucked it out the window, so it's likely that your sister never looked in there.

You frantically move the boxes and assorted clutter around in the attic, until you find where you hid the box the amulet was in. Digging around it's contents (you don't want to risk trying anything on blind, you got lucky - well, unlucky - last time) you come across a piece of paper with a picture of the amulet. You read:

Amulet

Origins: Unknown

Rank: Dangerous

Counters: There is a spell that renders the caster immune to its effects. Have included one set of ingredients for this purpose. Ingredients are one-use only, and hard to procure. Use wisely. Details are as follows...

This is it! This is your answer! You quickly rummage through the box and find the ingredients listed.