Calibre using Heuristics processing sometimes works. Sometimes the right xpath regular expression will do it. You can quickly view the code of your book in the Search and Replace Wizard to see what it is so you know what regex will work if Heuristics processing fails to find the chapters.
Make a backup of your mobi file so you can drag into Calibre each time you need run a test conversion.
I don't use Sigil at all.
