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Do you mean the Belarus -> Russia attraction? And attraction, meaning "romantic attraction"? From canon perspective, Belarus has had an affinity toward Russia since they were children (she was jealous of Ukraine giving him her scarf), and has a cameo in the very first episode and chapter where she threatens and interrupts other countries she perceives to be disrupting her brother (e.g., England). Her main interactions with Russia come in episode 42, where Russia explains his relationship to his sisters, and remarks on the unfortunate affection Belarus rains down on him--and episode 43, when she grabs Russia from behind as he listens to France's radio show.

While there is evidence that they can get along when Belarus isn't pawing at him for marriage, Russia has not shown any canon tendencies of romantic affection toward his younger sister.

(This, providing the fact that I haven't been watching the anime/have not been reading the strips for a few months now OTL /still needs to watch episode...54 or something.)