I use either the writer2epub or writer2xhtml plugins to produce a basic epub from LibreOffice.
If the .odt document is well-formated (i.e. use of styles rather than "direct formatting") the above will produce an epub container with relatively clean xhtml code inside. By itself it is usually good enough for home use, if you're not too picky. I wouldn't offer it for sale as is, though.
Then I use sigil to tweak and polish the epub into the final form I want. (E.g. adding the cover, extensive metadata, an in-line TOC, variants for kindlegen conversion, etc.)
In other words, use LibreOffice to produce the input to sigil, not to replace sigil.
Albert
|