Word's mail merge does not support one-to-many relationships. There are ways to coerce it, but only one of them can yield a table as a result and over the years it has become less and less reliable as Microsoft has not regarded it as important enough to maintain...
What you need to do is set up a query that provides ONLY the information you want displayed in the table, plus the key (SiteId). It's best to sort it so that all the SiteId entries list together, and are in the order the data will come through in the mail merge data source.
On the Insert tab go to Text/Quick Parts/Insert Field and select the Database field from the list in the dialog box. Click "Insert Database" and follow the instructions in the dialog box to link in the data. Be sure to set the Query Options to filter on the first SiteId from the data source. When you "Insert Data" make sure to choose the option to "Insert as a field".
This inserts a DATABASE field in the document which you can see by toggling field codes (Alt+F9). The field code can be edited and what you need to do is substitute the literal SiteId value you entered for the query with its corresponding MergeField.
When you execute the merge to a new document that should generate a table for each data record corresponding to the SiteId for the record. But, as I said, Microsoft hasn't done a great job of maintaining this, so it may require quite a bit of tweaking and experimenting.
If the results are not satisfactory then you should give up the idea of mail merge and use automation code to generate and populate the documents.
You can find more (albeit somewhat out-dated) information on this topic at http://homepage.swissonline.ch/cindymeister/mergfaq1.htm
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