Things can be both incompatible and related. For any given thing, the set of things related to that thing will typically be bigger than and encompassing of the set of things compatible with that thing.
I guess I don't really get your point. Is Clojure not a pure enough Lisp for your taste, or what? I've used Clojure and CL a similar (small) amount, and they seem incredibly more similar to each other than either of them to Fortran.
For me languages which are fully incompatible and allow no code sharing are not VERY related. Languages VERY related to CL are Emacs Lisp, ISLisp and some other Lisps in the Lisp 1 / Maclisp tradition.
