Your description suggests all you need is an ordinary letter merge, with a mergefield for each field in your data source, along the lines of:

Client Name: «Given_Name»«Family_Name»
Brother's name | Brother's Age
«Brother1» | «Age1»
«Brother2» | «Age2»
«Brother3» | «Age3»

where the words between the chevrons are your field names.

Note: You can't type or copy & paste the chevrons (i.e. '« »') - they're part of the actual mergefields, which you can insert from the mailmerge toolbar.

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Cheers,
Paul Edstein
[Fmr MS MVP - Word]