Hello guys,

see the pic attached. I have to level the floor, by sistering the existing joists, with 2×6 joists.
Old house, intermediate floor, some undersized joist, not access from below. To support the new subfloor, and to distribute the horizontal load, I will introduce solid blocking at the entire perimeter.

Question, would you install blocking and then sistering or vice versa? As you can see from the white builder line, sistering the joists would end up with solid blocking notched to fit the unlevelled size at each joist, introducing some possible inaccurate cutting that may create in the future squeaking due to movement (old joists thickness side is not levelled making the notch task challenging or labour intensive).

So, how would you proceed?

Practical experience and theoretical engineering inputs welcomed. 

I’m wondering whether I should install blocking