Note that in Schematron/Xpath you have to specify the steps of the path. You cannot say 'anyone who is a sibling of a sibling of a sibling arbitrary times is my sibling'. Xpaths are not them selves regular expressions that allow + on steps.
So this makes Schematron an 'expert system'. The next more powerful class of logic system would allow you to follow arbitrary chains (xpath does not allow searching/unification) that you might get if you ran Schematron on Prolog: however you can get much the same effect if you write an xslt function as a foreign element. (Or you can use brute force, and enumerate the xpaths to some satisfactory depth: sibking, siblingx2, siblingx3, siblingx4,....)
Rick