If you are using Sigil's auto ToC generator it should automatically create those indents/expansions.
Just use Tools/Table of Contents/Generate Table of Contents (Ctrl-T) and it will automagically create the ToC which you can then edit as needed.
It basis that creation on your proper use of heading tags <h1-6> to denote the hierarchy within the document.
for example:
Code:
<h3>THE OLD TESTAMENT</h3>
<h4>GENESIS</h4>
<h5>Chapter 1</h5>
<h5>Chapter 2</h5>
...
<h4>EXODUS</h4>
<h5>Chapter 1</h5>
<h5>Chapter 2</h5>
...
<h4>LEVITICUS</h4>
<h5>Chapter 1</h5>
<h5>Chapter 2</h5>
...
<h3>THE NEW TESTAMENT</h3>
<h4>MATTHEW</h4>
<h5>Chapter 1</h5>
<h5>Chapter 2</h5>
...
<h4>MARK</h4>
<h5>Chapter 1</h5>
<h5>Chapter 2</h5>
...
<h4>LUKE</h4>
<h5>Chapter 1</h5>
<h5>Chapter 2</h5>
...