Does it have to be spath? If you're open to using xpath instead you could do something like this:
| xpath outfield=MemberSid "//*[local-name()='Data' and namespace-uri()='http://schemas.microsoft.com/win/2004/08/events/event' and @Name='MemberSid']"
It is indeed a bit more complex of a query, thanks to unprefixed XML namespaces, and the corresponding xpath behaviour as pointed out in this stack overflow answer, but you can get the value of any data element with a specific Name attribute this way.
Dummy Test:
| makeresults | eval _raw="<Event xmlns=\"http://schemas.microsoft.com/win/2004/08/events/event\">
<EventData>
<Data Name=\"MemberName\">member</Data>
<Data Name=\"MemberSid\">ABC</Data>
<Data Name=\"TargetDomainName\">domain</Data>
<Data Name=\"MemberName2\">member</Data>
</EventData>
</Event>" | xpath outfield=MemberSid "//*[local-name()='Data' and namespace-uri()='http://schemas.microsoft.com/win/2004/08/events/event' and @Name='MemberSid']"
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