I asked myself, "Are there any fics out there that deal with JUST Kuno 
and Kodachi?" No, wait, let me refine that: 

1. A fic about the Kuno siblings. 
2. Neither Kuno nor Kodachi are homocidal or insane/mentally ill. 
3. The story isn't about boyfriend/girlfriend stuff (that just plain 
gets one nowhere). 
4. The story touches on their 'strange' relationship (ie, Kuno behaves 
like a dad toward Kodachi sometimes). 
5. Tries to explain rationally why the Kunos behave as they do (I tried 
myself, if you were wondering... shameless fic plug)

It doesn't quite fit all your criteria, at least not by itself it doesn't,
but check out Richard Lawson's "The Dying of the Rose" (I think that's the
title -- Richard, please correct me if I've bungled it), which is part of
his "Thy Inward Love" cycle.  You can get most of what you want in #5 from
TIL itself, but the focus of this incredible and excellent story is mental
illness and recovery therefrom, so if #2 is important to you, you may not
care for it.

-- Bob

-- 
===============================================================================
Robert M. Schroeck            || "When in trouble or in doubt,
rms@cnj.digex.net             ||     Run in circles, scream and shout."
http://www.cnj.digex.net/~rms || I have no mouse and I must scream.