A sister is relatively easy, cheap and simple to fix a ubiquitous problem in an old brownstone building. You could use 1/4″ steel plate, but that is also a fussy detail to work with and more expensive than an LVL. I have no idea why plumbers were so determined to cut and notch so many joists to plumb a building, but they more than any other trade, sufficiently compromised the structural integrity of our building. As a result, we have had to deal with the implications of their zeal and ardor for removing structure in favor of piping.
Above you can see our brand new orange sister nestled against a joist that was notched for the toilet waste pipe. There was about 1″ of the original joist left, which is why the floor in the bathroom had become soft. The structural redundancy in old brownstones is impressive, but so is the notching that often leads to sloping and sagging floors.