Is deck joist sistering a sloppy repair?
Sistering board on the exterior is problematic.  Without a gap between the two boards, water gets trapped and rotts them from the middle out.  Same reason ledgers without spacers are a problem spot on houses.  If you sister the joists, make sure you put in a spacer to allow air flow between the two boards.

Or do not sister and just install another joist nearby.  Not pretty layoutwise from the bottom, but not really noticeable from the top for the layman.