I have found that her grandfather Prince Nicolae Sturdza married 3 times...firstly to Princess Marghiolita Ghika-Comanesti in 1824,secondly to C.Sturdza and thirdly to Maria Rosetti-Roznovanu which could mean that Maria as a third wife and presumably younger then her husband was born after 1799 which would make Anica Bogdan her mother and Natalia's great-grandmother presumably that Maria's father was Prince Iordache Rosetti-Roznovanu which I find highly likely due to the fact that wife of Prince Ioan Sturdza 1761-1842,who ruled in Moldavia from 1822-1828 was Princess Ecaterina Rosetti-Roznovanu,who lived from 1764-1847 and from the same Roznovanu line...
If something will help your search I found that Ioann Keshko was mayor of the town of Kishinev or Chişinău in Romanian and later president(Marshal as the call it)of the nobility of Bessarabia while his wife Natalia Bals died in 1830 of cholera...you can also find his name in the stone of Catholic church from 1840 in Kishinev under the name of "Joanne Keszko" written in Latin...
http://www.bessarabia.ru/k-k3.jpgI also found a very interesting fact that direct ancestors of Queen Natalia were Constantin Keshko and Irina Buzny(Бузни in Russian),daughter of Ilya Buzny who took his wife's family name...Irina Buzny was sister of Konstantin Buzni who is a direct ancestor of senior albeit collateral branch of Karadjordjevic family...
Line of Iliya Buzny to Natalia Keshko and Obrenovic family:
Iliya Buzny,born around 1700=Klotshera Buzny
I
Irina Buzny=Konstantin Keshko
I
__________=__________(could be 1 or 2 generations empty until Ioann Keshko-don't know)
I
Ioann Keshko,after 1840=Natalia Bals(1812-1830)
I
Peter Keshko(1830-1865)=Princess Pulcheria Sturdza(1831-1874)
I
Natalia Keshko(1859-1941)=King Milan of Serbia(1854-1901)
Line of Ilya Buzny to Aleksa,Claimant to the Serbian Throne and Karadjordjevic family:
Iliya Buzny,born around 1700=Klotshera Buzny
I
Konstantin Buzny(1740/1750-1821)=Smaranda,of a greek noble family
I
Viktoria Buzny,died in 1824=Nikolai Trokin-Marshal of the Russian Court
I
Maria Trokin(1806-1827)=Aleksa Karadjordjevic(1801-1830)
I
Prince George Karadjordjevic(1827-1884)=Sarka Anastasijevic,daughter of Misa Anastasijevic,one of the richest merchants
I
Prince Aleksa Karadjordjevic,Claimant to the Serbian Throne(1859-1920)=Myra Abigail Pankhurst(1859-1938)
If you want to know more about maybe Keshko or Kesko ancestors try to write an e-mail here because they say it is possible to know a family tree from 300 years ago until today:
http://nistru-prut.info/index.php?mode=ngenprez&prez=gen&lang=eng&namegen=Kesco.