Add sisters.

An attempted rebuild/repair would be more work and just as costly as new stairs. Visit a box store or real lumberyard (preferable) and pick up a three pack of pressure treated 5 raise stair stringers and replace what is there.

"An attempted rebuild/repair would be more work and just as costly as new stairs."

Not even close. The chances of getting pre-made stringers with the correct rise/run are astronomical.

Sister away, please.

Remove existing stairs, clean out all that loose dirt, compact the base, and provide a concrete slab to support newly cut stringers.

Here is an example of the same situation

Good luck