You only need calibre:series and calibre:series_ index for epub2. Epub3 has its own official way to handle series and series_index:
Here is a snippet from the opf showing how to do that.
Code:
<meta id="seriesid" property="belongs-to-collection">NAMEOFSERIES</meta>
<meta refines="#seriesid" property="collection-type">series</meta>
<meta refines="#seriesid" property="group-position">INDEXVALUE</meta>
In Sigil's metadata editor you do the following:
1. Add an Element and select "Belongs to Collection", fill in series name
2. Add a Property to that Element and select "Collection is a Series"
3. Add a Property to that Element and select "Position in Group", fill in series index
Sigil's MetaEditor will properly create the seriesid and all the refines needed.
Why the epub3 spec took something so simple and made it so complex is anyone's guess!