Island looks like it should be related to isle, but the story is more complicated. In earlier English, the word was often written as iland, without the s.
The extra s appeared later because scholars and writers connected the word with Latin insula, which also gave English isle. That connection seemed sensible, but it was historically wrong.
The English word island comes from Old English iegland or related forms. It is built from elements meaning something like water and land. Over time, pronunciation and spelling changed, and iland became the common form before the silent s was inserted.
- island: from Old English roots, once commonly iland
- isle: a different word in form, borrowed through French and ultimately from Latin insula
So the modern spelling preserves a learned mistake. The s stayed in writing, even though it is not pronounced. That is why island and isle look related today, but they did not develop in the same way inside English.