The terms all reflect the underlying Xml structure:
<root>
<record childattribute="value">
<childfield>value</childfield>
</record>
<siblingrecord>
<childrecord>
<childfield2>value2</childfield2>
</childrecord>
</siblingrecord>
</root>
Sibling is equivalent to brother or sister i.e. at the same level
A good idea may be to learn about Xml.
W3C has the spec, and there are tutorials, etc http://www.topxml.com/xml/LearnXML.asp
One of the keys to Biztalk is understanding Xml Namespaces.