I don't know many of the details concerning Mac's new macOS, but I assume that since they offer a free upgrade to it from El Capitan, that the previously installed programs will continue to work with Sierra. So I'd expect the latest version of Sigil (v0.9.7) to work fine. I'll let a Mac expert say for sure, though.
The latest version can always be found
here (downloads are attached to the bottom of the post).
You want the "Sigil-0.9.7-Mac-Package.dmg" download.