Place a cement block directly below the bowed joist, centered with the lowest point. Set a house jack on top of the block. Measure the distance between the top of the jack and the bottom of the joist.
Cut a 2-by-6-inch board to the measured length with a circular saw. Set the board on top of the house jack. Pump the jack until pressure holds the board in place. Hold a 4-foot level on the bottom of the joist as you pump the handle of the house jack. Stop pumping the jack handle when the joist is level.
Set an open tube of construction adhesive in a drop-in caulk gun. Apply a zigzag bead of adhesive to one side of the bowed joist. Place a 2-by-10-inch board against the glued side of the joist. Hold the joist in place by placing one c-clamp every 2 to 4 feet.
Secure the sister joist to the joist that has bowed by hammering 3 8d nails in a line every 16 inches. Keep the top and bottom nails 1 inch from the top and bottom edge. Place the third nail 5 inches from the bottom edge of the joist.
Remove the c-clamps from the two joists. Lower the house jack while holding the 2-by-6-inch board with one hand. Remove the board, house jack and cement block from the area.