Combine first 6 ingredients, 1/4 teaspoon chili powder, and 1/4 teaspoon sugar in a small bowl; stir well.
Let mixture stand at room temperature for 2 hours.
Combine remaining 1/4 teaspoon chili powder, water, vinegar, and remaining 2 teaspoons sugar in a small skillet; bring to a boil.
Add nectarine halves, spoon liquid over fruit.
Cover, reduce heat, and simmer 5 minutes or until nectarines are tender.