Antonyms of straighten:
bend, flex, arc, bow, hook, round, entwine, kink, swirl, turn, twine, twist, coil, loop, spiral, wind, crook, curl, curve.
Examples of usage:
  1. Some daylight flowed in through a slightly open fire exit and she caught at a last moment of darkness to straighten her hat.
    "Star-Dust A Story of an American Girl", Fannie Hurst.
  2. If anybody can straighten things out, she's the one, though I don't see how it's goin' to be done.
    "The Princess and Joe Potter", James Otis.
  3. I'm helping straighten it!
    "This World Is Taboo", Murray Leinster.