How to sister joists to level a floor when they've already been sistered to level the ceiling below?

Question

We are remodeling a second-floor bathroom. The floor dips, and my plan was to sister the joists to level the floor.
This is an old house, built in 1920. The previous owner redid the kitchen. The bathroom we are remodeling is above the kitchen.
I started pulling out the tub last weekend, allowing me to see into the floor and I noticed that the joists are already sistered , presumably to level the ceiling of the kitchen below. So I'm not sure whether I can even level the floor via sistering.
Should I get/cut narrower boards and rest them on top of the existing sisters or sister again on the other side of the joists? Or is there something entirely different I should do to level the floor?
Supplement
Given questions in the comments about why there's such a severe dip, I am attaching a photo of a sistered joist under the kitchen floor showing a split joist, which I assume is the same reason the bathroom floor has a 1-2" dip.

Also, I tipped the old tub on its side, which allowed me to share pics of these joists:

Jack · Answer

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.

How to sister joists to level a floor when they've already been sistered to level the ceiling below?

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