Heat four 8-ounce jars in simmering water until ready for use. Wash lids and rings in warm soapy water.
Combine peaches, sugar, apple, lemon juice, and water in a large pot over high heat. Bring to a boil, stirring constantly. Cook until peaches soften and mixture thickens, about 20 minutes. Mash peaches with a potato masher. Continue cooking until a small amount of jam dropped on a plate stays in place and doesn't run, 5 to 10 minutes more.
Remove apple wedges and seeds with a slotted spoon.
Pack jam into hot jars, filling to within 1/4 inch of the top. Wipe rims with a clean, damp cloth. Top with lids and screw on rings.
Place a rack in the bottom of a large stockpot and fill halfway with water. Bring to a boil and lower in jars using a holder, placing them 2 inches apart. Pour in more boiling water to cover the jars by at least 1 inch. Bring the water to a rolling boil, cover the pot, and process for 10 minutes.
Remove the jars from the stockpot and place onto a cloth-covered or wood surface, several inches apart, until cool, about 12 hours. Press the top of each lid with a finger, ensuring that lid does not move up or down and seal is tight.
