It is a bad habit to get hung up on ordered lists. It inclines some people to change
a. This is some text
b. The is more text
into
1.This is some text
2.The is more text
Also some conventions like a start tag if you have a page break are not proper in epubs.
To get a nested appearance you can use <p> tags having differnt margins in the stylesheet. Like
.firstlevel {margin-left:5%;}
and make a second with a larger left margin percentage and so on. Such as <p class="firstlevel">
To automate it you can use the clips function in Sigil and then just right click to insert the proper level into the paragraph tag.