I also doubt any of them will live in Greece. They are all primarily British-Americans and have little meaningful connection to Greece other than spending holidays there as tourists. It is sad that the family has given up on Greece.
EDIT: As I understand it,
1. Marie-Chantal was born in London before 1983, so she is a British citizen by birth regardless of her parents' nationality. She is also probably a US citizen by descent having been born to an American father who satisfies the minimum requirements of time of residence in the US prior to Marie-Chantal's birth and was married to her mother at the time of the birth.
2. Maria-Olympia, Konstantinos-Alexios, Achilleas-Andreas, and Aristidis-Stavros were all born in the United States, so they are US citizens by birth under the 14th amendment. They should also be British citizens by descent having been born outside the UK after 1983 to a mother who is a British citizen otherwise than by descent.
3. Odysseas-Kimon was born in London after 1983 to a mother who is a British citizen, so he is a British citizen by birth. He might be also a US citizen by descent having been born outside the United States to a mother who is herself a US citizen, but Marie-Chantal would need to satisfy the minimum requirements of length of residence in the United States.
That young woman has so many romantic options. The world is literally her oyster.
