As ndrosen has pointed out, the corresponding Standard German expression is "(Es) macht nichts".

Nix is common even in colloquial German. This could also be due to the use of nix (ultimately from the German) in American slang. I'm assuming the /xt/ of macht was altered to /ks/ by analogy with it.

("Idiolect" is the word for one's own specific habits of speech; there ought to be one for a family's.)

I've seen "oikolect" used for this purpose but it's not a standard term.