Exactly as it says, Belarus learns somehow that she is not Russia and Ukraine's sister, but their daughter. Her reaction is up to anon.

note: Russia and Ukraine are still brother and sister. Reason why they had sex in the past is also up to author (but no rape, please!). They do not date now.

And I'd like to see sane!Belarus, though maybe with a bad temper, her obsession with Russia was more like teenage crush.