How to sister joists to level a floor when they've already been sistered to level the ceiling below?
There should be no reason you cannot sister the other side with new material. You could use 2X6s to get enough surface for good attachment, notching a little here and there to get around wires and such. If the original joists are clean enough, construction adhesive can be used to minimize the need for so many fasteners.
Yes you can rip material to add to the top of the joists too, it is a little more tricky to a degree. glue and screw it in place too. the cut may need to be curved a bit yo accomodate th bow of the original joists. A circular saw will have no problem doing a slight curve.