$ cat test.py
str = """<?xml version="1.0" encoding="UTF-8"?>

<network>
    <description name="Boston">
        This is the configuration of our network in the Boston office.
    </description>
    <host name="agatha" type="server" os="linux">
        <interface name="eth0" type="Ethernet">
            <arec>agatha.example.edu</arec>

            <cname>mail.example.edu</cname>
            <addr>192.168.0.4</addr>
        </interface>
        <service>SMTP</service>

        <service>POP3</service>
        <service>IMAP4</service>
    </host>
    <host name="gil" type="server" os="linux">
        <interface name="eth0" type="Ethernet">

            <arec>gil.example.edu</arec>
            <cname>www.example.edu</cname>
            <addr>192.168.0.5</addr>
        </interface>

        <service>HTTP</service>
        <service>HTTPS</service>
    </host>
    <host name="baron" type="server" os="linux">
        <interface name="eth0" type="Ethernet">

            <arec>baron.example.edu</arec>
            <cname>dns.example.edu</cname>
            <cname>ntp.example.edu</cname>
            <cname>ldap.example.edu</cname>

            <addr>192.168.0.6</addr>
        </interface>
        <service>DNS</service>
        <service>NTP</service>

        <service>LDAP</service>
        <service>LDAPS</service>
    </host>
    <host name="mr-tock" type="server" os="openbsd">
        <interface name="fxp0" type="Ethernet">

            <arec>mr-tock.example.edu</arec>
            <cname>fw.example.edu</cname>
            <addr>192.168.0.1</addr>
        </interface>

        <service>firewall</service>
    </host>
    <host name="krosp" type="client" os="osx">
        <interface name="en0" type="Ethernet">
            <arec>krosp.example.edu</arec>

            <addr>192.168.0.100</addr>
        </interface>
        <interface name="en1" type="AirPort">
            <arec>krosp.wireless.example.edu</arec>
            <addr>192.168.100.100</addr>

        </interface>
    </host>
    <host name="zeetha" type="client" os="osx">
        <interface name="en0" type="Ethernet">
            <arec>zeetha.example.edu</arec>

            <addr>192.168.0.101</addr>
        </interface>
        <interface name="en1" type="AirPort">
            <arec>zeetha.wireless.example.edu</arec>
            <addr>192.168.100.101</addr>

        </interface>
    </host>
</network>
"""

sel = "/child::network/child::host/child::interface/following-sibling::interface/parent::host/attribute::name"

from lxml.etree import fromstring
x = fromstring(str)
print x.xpath("/child::network/child::host/child::interface/ancestor::host/child::interface/following-sibling::interface/preceding::host/attribute::name")

$ python test.py
['agatha', 'gil', 'baron', 'mr-tock', 'krosp']