I'm with SpoonySage on this. Mario is a Donkey Kong character.
But on a broader level, they each exist in the others' universe. If you drew a ven diagram with one circle for Mario characters and another for Donkey Kong characters, both Mario and Donkey Kong would be in the overlap.
I'd say so. Since Rare is owned by Microsoft Donkey Kong was still able to stay with Nintendo. That alone should be proof enough.