This is a portrait of Catherine and Mary Radziwill. The Radziwill's were a noble family of Lithuanian origin. There were many notable figures from this family in Belorussian (as well as other neighboring countries) history. Although I didn't find anything specific about Mary Radziwill, Catherine had her own Wikipedia entry.
More cute stamps from their wild animal series.
{BY-496165}
Postcrossing card #75