Ed, one method would be to "sandwich" rather than sister; an excellent stiffener without the bulk.
This is done by laminating sheet metal between one side of the joist and 3/4" plywood. The sheet metal and the plywood should be cut to full width of the joist, with all butt joints fitting tightly, and a thin layer of adhesive between each layer is good, too. Use screws (preferred) or nails for final attachment.
A bit of a job to do in a crawlspace, but does work very well.