I picked up a 12 string acoustic like that years ago.
I had no absolutely no idea how to straighten it but it was a trade in for something else and had only cost about £20 in real terms.
[b][u]This is what I did, it's not necessarily the right way to do it though.[/u][/b]
I took off the machine heads and slackened the truss rod right off.
I then filled a tall pedal bin full of water and immersed the neck and then left it for a week.
I made two wooden neck braces (one flat, the other with a contour cut out to accept the back of the neck).
I used half a dozen C clamps and then tightened the whole thing up, making sure I put plenty of pressure on the opposite side of the twist (essentially persuading it to pull back the other way).
I then stuck it in the airing cupboard like that for about three weeks.
Once a week I'd take it out and tighten up the C clamps again.
Restrung it, put a new set of strings on, retightened the truss rod and set it up.
I've still got it, still play it and the neck is as straight as an arrow.