Thanks for giveing such a good example of what you want to do.
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click on person Y
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click the "Add Partner/Ex" on the left
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click on "Partner with someone already in tree"
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pick person A's brother
Note that this will probably lead to a "duplicate" box in your family tree. This is not a problem. It is just Family Echo's way of designating that this person is related in multiple (duplicate) ways.