Combine rhubarb, sugar, raspberries, and lemon juice in a large pot. Let sit until sugar begins to dissolve, about 5 minutes. Bring to a boil; cook, stirring often, about 15 minutes. Add raspberry-flavored gelatin; stir until dissolved, 1 to 2 minutes. Remove jam from heat.

Pack jam into hot, sterilized jars, filling to within 1/4 inch of the top. Wipe the rims of the jars with a moist paper towel to remove any residue. 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 place jars 2 inches apart into the water using a holder. Pour in more boiling water if necessary to bring the water level to at least 1 inch above the tops of the jars. 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 completely cool, about 2 hours. Press the top of each lid with a finger, ensuring that the seal is tight. Store in a cool, dark area.