@Brotdose: As DiapDealer has already pointed out, the rendering of an .ncx based TOC, depends on the reading system and very few of them handle other RTL features correctly.
If you create an ePub3 book you could theoretically style the nav xhtml file (the epub3 equivalent of the ncx file), however, since even the official IDPF Readium ePub3 reference app ignores styles in nav xhtml files this is pretty much pointless.
BTW, you can download an ePub2-compatible
Arabic ePub3 test file from MR.
tl;dr: You can't style the TOC entries show in the Table of Contents window in Sigil. You could theoretically style an ePub3 TOC, but most reading apps will ignore the styles.