Querying "the other side" of an association

Here's a SQL query created through List Manager that I'm working with:

Select Distinct a.customer_no
 From t_customer a (NOLOCK)
JOIN vxs_xref e (NOLOCK) ON a.customer_no = e.customer_no
 Where  IsNull(a.inactive, 1) = 1
 AND a.created_by in  ('webAPI')
 AND a.cust_type in  (9)

Every one of the records I'll be pulling has a single association to another record (we're managing some child/adult records here). Adding onto the above code, I can easily select records that have certain properties in their associations. For example:

 AND e.type in (1)

But this looks only at the "side" of the association present on the record (because associations of course come in pairs, with one side created automatically when you manually create the other side).

So, how do I query that other side? If the above code pulls a list of child records, how do I query the associations to those child records?

Do I make sense? It's so hard to type this stuff out sometimes :-)