How do I reinforce the ends of 2x8s that are sistered to 2"x5" (actual size) joists?
I'm reading this as the office is over the crawl space and you want to add the 8" joists so that the floor height is unaffected.

You say 


  I will have to reduce the height of the new joists from 7.5" to 5" at the ends.


I say you don't, and if you do, it will compromise your attempt to strengthen your floor.

Get joist hangers:



Instead of sistering your joists to the old 5", hang the new 8" (ie: 7.5") joists halfway between each 5" joist.  Put them tight up against the floor and secure them with the joist hangers (again, tight) and that will reinforce your floor quite nicely without compromising the integrity of the board by cutting them down to 5" at the ends.