AFAIR, calibre only reads the metadata from an epub on import so it would not see any changes you make using Sigil unless you remove and re-import the epub.
Generally, my workflow is to use calibre to update the metadata, cover images, etc. and ModifyePub to embed them in the epub. I then use Sigil to edit the epub (I could use calibre's editor but I find some tasks are easier using Sigil and it's plugins).
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