1880 victorian front stair repair/rehab question. Old carpet removed from stairs which lead to the discovery of the tread and riser separating and sagging from the skirt board. The stairs are solid and comfortable. The separating/sagging area is only where the staircase bends. 

I have already addressed some structural issues in the basement on a main beam which does support the staircase at the second floor. 

Question is how to go about fixing this issue or if it’s worth the hassle? 

Any ideas are welcome!!