Place raspberries in a blender. Cover and blend until smooth. Press pureed berries through a fine-mesh sieve; discard seeds. Measure 1 cup raspberry puree.
In an 8- to 10-quart heavy pot combine raspberry puree, peach nectar, and lemon juice. Stir in sugar. Bring to a full rolling boil, stirring constantly to dissolve sugar. Quickly stir in pectin. Return to a full rolling boil, stirring constantly. Boil hard 1 minute, stirring constantly. Remove from heat. Quickly skim off foam with a metal spoon.
Ladle hot jelly into hot, sterilized half-pint canning jars, leaving a 1/4-inch headspace. Wipe jar rims; adjust lids and screw bands.
Process filled jars in a boiling-water canner 5 minutes (start timing when water returns to boiling). Remove jars from canner; cool on wire racks.