The sister joist was not installed properly and the floor is sagging; trying to convince boss it is truly an issue that simply "hitting the sister joist with a sledge hammer" won't fix..please send me in the right direction. Need to be able to quote something more official than my opinion that the floor needs to be looked at by a structural engineer or, at the very least, the floor jacked back up and the joist replaced.
I don't live in Arkansas, but Google says your building department uses the 2006 International Building Code (IBC) with amendments. (Amendments are added to all building codes by the local Building Codes Division in each state to reflect local conditions, etc.)
You say it's a "home" so the "loading requirements" are usually somewhat less than commercial or industrial loading. So, if the joist failed, (you didn't say how it failed,) then you need to investigate the "why", because this is fairly uncommon...especially if it's one joist in a series of "like joists".
While the Building Code states a "Uniform Load" be applied throughout the house, we'll often add a joist or two under refrigerators, spa tubs, etc. because we know there's an extra load in those areas.
I think Ed Beal is correct that you can sister a joist on each side of the failed joist and fix the problem. However, I'd recommend you find the cause. If you suspect all the joists in this area are undersized, then you need to have it reviewed. Here where I live, you could call the local Building department and tell them the "span" and the "spacing" and they'd let you know if it meets code. If it's an engineered joist, you can call the manufacturer's rep and he can tell you.