ORIGINAL: Zor
I assume by guessing that the covering is a heat shrinkable film that logically needs more heat at the wing tips to stretch the film over more compounded curves thus explaining the tip warps.
Reheating the wing tips while twisting the wing tips in slightly opposing direction should remove the warp. Go at it gently till no warp is discernable.
Just to add onto Zor's advise,
If the warp is built into the wing, not just the covering, and being held into place by the spar forward sheeting, then just twisting and re-heating might not do the trick 100%,, sure worth a try before preforming surgery though.