> Sorry if this is too simple, but I'm still a bit new at this. 
> I need to compare the relative positions of two preceding 
> siblings and grab the value of the nearest one.

I haven't managed to follow the detail of your requirement, but given two
nodes $A and $B you can select the one that comes last in document order
using

($A|$B)[last()]

or if you prefer

if ($A>>$B) then $A else $B

Michael Kay
http://www.saxonica.com/