Is it foam core? If so, it won't take to twisting.
Are you sure the limb wasn't just made a little less than straight? A longbow limb will bend and twist toward the weak side if too much material was taken off one side of the limb, particularly toward the tip of the limb. If such bends aren't severe, you can straighten them out by sanding the edge of the limb opposite the bend, i.e., if it's bending/twisting toward the left, sand the right edge of the limb where it's bending.
If it's a 3-piece bow, take the limbs off and lay them limb edge down on a flat surface to check for straightness and if too much material may have been taken off one side or the other. This is a good way to also determine exactly where to sand the limb on the off side to straighten it. Works for a one piece bow as well, but more difficult to get the limbs flat on the surface.
One other thing to check. The nocks. If the string nock is cut deeper on one side than the other, it can twist the end of the limb out of alignment. The solution is to cut the opposite side string nock deeper to match (or learn to live with the slight bend).
I've seen both of these construction errors even on high priced custom bows.
I think it's much more likely that the limb was made crooked than that it developed a twist later, But If the limb is actually twisted, and if you feel you must straighten it, and If it's wood or bamboo, you can get more leverage on the limb by building a jig/wooden wrench to grasp and twist the limb.
Take a piece of 1x4 about a foot to foot-and-a-half long and cut a horizontal slot (big enough for the limb to fit into) about 1-2 inches from the end of the board. Slip the limb into the slot and grab the lower end of the board/wrench to twist the limb. You may want to clamp the bow in a vice before doing this. Might also be a good idea to heat the twist area with a hair dryer before trying to twist it back into shape.
I don't recommend this procedure. Because of the tremendous force you can apply with the "wrench," it's quite easy to break the limb. Good luck.