Makes about 10 half-pints. Wash and peel the lemons thinly; remove the white pith and seeds and discard them. Cut the peel into fine shreds; reserve the pulp. Simmer the peel with 2 cups of the water in a covered preserving kettle for about 1 hour, or until tender. Drain the liquid from the lemon shreds and set both aside.
Wash the grapefruit and cut it and the lemon pulp into small pieces; cover with the remaining 10 cups of water and simmer gently in a covered preserving kettle for 1 1/2 hours, or until the fruit is tender. Add the liquid from the lemon shreds to the kettle and bring just to a boil. Remove from heat and strain mixture through a jelly bag. Measure the juice into the preserving kettle and add 1 cup of sugar for each cup of juice. Add the reserved lemon shreds and over high heat, boil hard for about 15 minutes until a jelly thermometer read