Check if XML element exists (BPEL)

I'm trying to check if an XML element exists in BPEL 10g.

Sample:

<soap:Envelope xmlns:soap="http://ift.tt/sVJIaE">
    <soap:Body xmlns:ns1="http://ift.tt/1JLW4fw">
        <ns1:Person>
            <name>John</name>
            <ns1:Brother>
                <bName>Paul</bName>
            </ns1:Brother>
        </ns1:Person>
    </soap:Body>
</soap:Envelope>

Here I'm trying to check wether <ns1:Brother> exists. I've tried using the boolean() function as:

boolean('/ns1:Person/ns1:Brother') 

The problem is that it's always returning true, even when the element Brother does not exist.

What should I do to solve this?