Old English twilic described a double thread and partly translated Latin bilix. Middle English twyll continued toward twill.
The first arrow is partial translation, not an ordinary sound change. Twylle is an alternative Middle English spelling.
Sources
- Merriam-Webster: twill
Latin bilix supplies a partly translated model for Old English twilic, followed by Middle English twyll and modern twill.
- Online Etymology Dictionary · twill
Meanings of the older forms on the word path.