She was the granddaughter of Olimpia Maidalchini, grand-niece of Pope Innocent X and wife of Maffeo Barberini, Prince of Palestrina.
Maidalchini, realizing her influence over Pope Innocent was waning, arranged the marriage to facilitate a return of the Barberini to Rome in order to curry favor with a number of the Barberini family's cardinals.
The Barberini, too, were keen to return to Rome and were enthusiastic about Maidalchini's plan to marry Pope Innocent's grand-niece, Guistiniani, into their family.
After the ceremony, however, the child refused to go home with her new groom to allow the marriage to be consummated.
Her mother, who had barely been involved in her upbringing, appealed to her by telling her that other girls were forced to marry decrepit old men and that, by comparison, Barberini was a far better option.