If you are currently maintaining your source files in LaTeX, I would think you might want to convert them directly to EPUB without bothering with Sigil.

Or simply use Sigil to preview the auto-generated EPUB.
It looks like Plastex already has an EPUB renderer.
You could also use pandoc.
If you want, though, Plastex is written in pure python, so you could turn it into a Sigil plugin and use it to import a LaTeX book into Sigil.
EDIT: Beaten by DiapDealer.