[HayZee518]

normal new construction floor joists are 2x10x [whatever length] to keep from twisting X type bridging is installed between the joists every 8 feet of the span. The joists are installed 16 inches on center. The crown of the joist is installed upward so it there is any "sag" it will impart a downward and outward pressure from the center. It seems odd that a joist would sag - say - four feet from an end unless the joist was green and there were several knots in the board at that end. if you have access to the supporting beam, when you sister a joist, see if you can install a double wide joist hanger. this will support the ends and stop some of the twisting at the ends. Then use carriage bolts in a staggered pattern and bolt them up. Install bridging like I said above.