[DiapDealer]

You already added similar attributes to xhtml files, so this would be very similar. Every epub has an *.opf file (usually, but not, always named content.opf). It should be visible in Sigil's Book Browser with the rest of the files.

Once you find the opf file, you double click it to open it in Code View. And much like adding the attributes

lang="he"

and

 dir="rtl"

to your html tag, you simply also add

page-progression-direction="rtl"

to its (the opf file's) spine tag.