Last summer I had to make that exact repair in the corner of my 22x24 storage garage. Since the sill board was rotted and the corner didn't drop any, I just cut the sill plate at the next clean accessable area between two studs, scraped out all the bad stuff and slid a new piece of PT into place. Toe nailed it, finished business. Simple job really. If there is weight on the corner post, nail a 2x6 a couple feet off the ground to it and an adjacent stud, cut a 2x4 to prop up the corner, hammer in the bottom to relieve the pressure on it if necessary. Not to hard a job. Looks like one of the corner studs has been spliced at one time already. You should findthe source of the water and fix that also or you'll be doing this again sometime in the future.
