http://somefitnessesite.com/?namesThis returns a list of all page names at the top level of the FitNesse site. Note that it does not return any of the names of the subpages.
FitNesse is a hierarchical wiki. Most wikis are not hierarchical. The spec for SisterSites does not provide for hierarchy in a wiki. It can't deal with the dots in the page names.
If you think about it, a hierarchical wiki is just a tree structure of flat wikis. That means that each level of the hierarchy can be a sister of some other wiki. Thus, if one wiki wants to cross reference to the FrabDaz project on http://fitnesse.zapadoodle.com it might request the sister page names by invoking the following URL: