If Poland was Russia's cousin, then he still is (or one of them died).
Poland and Russia are both Slavic countries, so they should be indirectly related somehow. I'm surprised there is a "German family" (which tosses together the German speaking countries), an "Asian family" (which tosses together a bunch of completely different cultural and ethnic groups into one, Asians are not all the same), and a "Kirkland family" (which forgot about the natives. Perhaps the natives were somebody else?). However, there are different Slavic groups, so I wouldn't question that... I'd say that in real life it'd make sense if they were cousins, but in canon material, it is not known.