Complex sentence: She is mentioned in the "Gustynskaia Chronicle" cousins ("second sisters") with Sophia Palaiologina of Muscovy, and was probably a niece of Trebizond's reigning couple, Emperor David and Empress Maria.
Split into simpler sentences: She is mentioned in the "Gustynskaia Chronicle" cousins ("second sisters") with Sophia Palaiologina of Muscovy. Through this connection, she was probably a niece of Trebizond's reigning couple, Emperor David and Empress Maria.