If you are going to discontinue using it, then I would stuff some insulation in the hole and then screw a piece of drywall over it as a fire stop. If it is going to remain active, then I would get a sheet metal boot to slide into it. As far as sistering the joist, you'll want to get a new piece in on top of the sill plate at the exterior wall. Make the sister as long as possible. Glue it to the original with some construction adhesive and then screw it fast with some structural screws. Some hefty C-clamps will help hold the two together until you get the screws run in.