I'm looking at this specifically as a release candidate for the Tolrea setting, but I don't think that it is necessarily so specific to what I'm doing there as to make it unsuited to use in some other setting. Thoughts on balance? Is there anything unclear about how the feat works?
Sympathetic Magic [General]
You have studied the eldritch resonance between like and like, and have developed techniques that allow you to focus better on the object of your magical techniques.
Prerequisites: Caster level 1st or greater.
Benefit: To use this feat, you must possess a sample of a creature's body (commonly a cachet of hair, nail parings, blood, saliva, or some other bodily fluid or emission). This sample acts as a focus component when you cast a spell against that creature, granting a +4 bonus to the Difficulty Class of the saving throw to resist your spell's effects. You may only use this feat against one creature at a time (thus, a fireball spell cast using this feat would gain an improved Difficulty Class against the Reflex saves of only one creature; any others in the area of effect save against the normal DC).
Special: If you possess a sample from the body of a creature of the same species as your target, you may use the sample as a material component to gain a +1 bonus to the Difficulty Class of the saving throw of a single spell, chosen at the time of casting. If the sample is from a parent, child, or sibling of the target creature, this bonus increases to +2.