"that caused his sister to refuse my brother"

The Cathedral of the Annunciation, Moscow
Creative Commons AttributionThe Cathedral of the Annunciation, Moscow - Credit: Michael Patterson

 The Russian Orthodox Church forbids marriage between certain degrees of kinship, as do most religions. In the Orthodox faith this extends to forbid a sister from marrying her brother's wife's brother, or a brother from marrying his sister's husband's sister. Therefore Marya can only marry Nikolai if Prince Andrei and Natasha do not get married.