You could do it like this. First you pick the object and save it's x, y and uid to some variables. Next you use an inverted "pick by uid" to pick all the instances except the original instance. The you can use the "pick closest" condition as usual.
global number x=0
global number y=0
global number obj=-1
pick sprite at random
set x to sprite.x
set y to sprite.y
set obj to sprite.uid
[inverted] pick sprite instance by uid obj
sprite: pick closest to (x,y)
do something