C Major key's Sub Dominant relative key is F. But instead, could Fm key also be a Relative key ?
So if that works..
From c minor key, could F Major be a relative key also ?
In common terminology, keys may be 'related' in various ways and to various degrees. C major has a close 'relationship' with C minor. It's pretty close to F major, G major. It's hardly 'related' at all to F# major (though I could confuse you with the 'b5 substitution' :-)
But in music terminology, 'Relative' has a specific meaning. It refers to the pair of major/minor keys that have the same key signature. C major and A minor, D major and B minor etc. Keep the word 'relative' for that.
(As @Tim commented, there may be translation issues here. But, in any language, I think there will be the concept of relative major/minor as a special type of relationship, with a special name.)
There may be an issue with translation here. Other answers address the relative keys of, e.g., C major and A minor, but I wonder if you're asking about closely related keys.
Traditionally, closely related keys are defined as keys whose key signatures differ by only one accidental. The keys closely related to C major are thus D minor (one flat difference), E minor (one sharp difference), F major (one flat difference), G major (one sharp difference), and A minor (no difference).
By this metric, F major and C major are closely related keys.
F minor, having four flats in the key signature, is thus not closely related to C major. However, there is a clear relationship between the two that allows one to smoothly move between them: C major is the dominant of F minor. So even if the two keys aren't "closely related," it's still pretty easy to switch between them smoothly.
in German I would say: jein! What means Yes and No.
But actually the answer is: no!
Relative keys have the same signature
but you are speaking about
Parallel minor and parallel major keys that have the same tonic
So fm as the minor subdominant in C is borrowed from the minor parallel of cm
and F as the major subdominant in cm is borrowed from the major parallel of C