2) Remove the skin from 2 of the Meyer lemons and slice them into 1/8" slivers. For the remaining Meyer lemons, remove the skin and the pith and discard. Working over the pot you intend to use for making the jam, and using a paring knife, supreme the lemon segments, reserving the middle membrane (essentially the core of the fruit). Put membranes from the grapefruit and Meyer lemons in a jelly bag (or doubled up cheesecloth) and tie closed.
3) In a wide and deep pot, combine the grapefruit & lemon segments, grapefruit & lemon peel, and jelly bag filled with the membranes. Add the lemon juice and enough water (or juice) to equal about 2 cups total. Simmer until the grapefruit & lemon peels are tender, about 20 minutes. Remove the jelly bag and set aside until cool enough to handle.
4) Working over a large bowl, squeeze the juice from the jelly bag. The juice will extract easily at first but keep squeezing and wringing it out until you extract 1/3 cup of pectin. Add the extracted pectin and the 2 teaspoons powdered pectin to the pot along with the 2 12# of sugar and the rose geranium leaves. Place over high heat and boil, stirring now and then, until marmalade is between 222 and 225 degrees and passes the plate test. (ie: spoon a little onto a frozen plate and put in the fridge for 3 minutes. If it thickens like jam, it's ready.) When the jam is done, fish out the rose geranium leaves and discard.
5) Meanwhile, put 6 sterilized 1/2 pint canning jars and lids on a baking sheet and place in the oven to keep warm. When jam is done, remove jars from the oven. Ladle jam into the hot jars, filling them as high as possible. Wipe the rims. Fasten the lid tightly. Process for about 15 minutes in a hot water canner. Allow the jars to cool until a seal is made on all the jars. Keep at room temperature up to 1 year.