I did a trilogy about Kuno and Kodachi, and just recently posted a
follow-up to that story.  I'll adress your points one by one.

1. A fic about the Kuno siblings.

Yep, although the first was just about Kuno.

2. Neither Kuno nor Kodachi are homocidal or insane/mentally ill.

The fourth part sorta hinted at this.

3. The story isn't about boyfriend/girlfriend stuff (that just plain
gets one nowhere).

Yes.

4. The story touches on their 'strange' relationship (ie, Kuno behaves
like a dad toward Kodachi sometimes).

Ditto

5. Tries to explain rationally why the Kunos behave as they do (I tried
myself, if you were wondering... shameless fic plug)

Yes, the whole trilogy was about that.  But the fourth part gave more
insight as to why they felt the way they did.

 I'm probably asking too much, but I mean, I haven't seen a single fic
out there where some sort of rational relationship between brother and
sister. I mean, I don't know about any of you out there, but they seem
to be the typical brother/sister duo, despite eccentricities.

Mine did.

 Or do I have an unusual Kuno bias (now THERE'S a thought! we have the
first one RIGHT HERE on the FFML! Come one, come all, the first guy who
likes BOTH Kunos!!! Waaaahaaaahahahahahahaha!!!) and am being
delusional? Heh. ^^;;;;

Nope.  People on the ML like Kuno, just some take it way to far (like me)

Hope I could help.
Cory Holmes
gholmes@rodeo.sd27.bc.ca