Steph: You are a bit misinformed about the family of Madame Xenia Sfiris, who among other things, attended the burial of Emperor Nicholas II in 1998 and provided the sample of mt DNA which helped to positively identify HIM.
You are correct that Xenia Sfiris had only one child, Tatiana Sfiris. Tatiana married twice. Her first marriage to Alexei Giannakoupoulos (I hope I got this name right) had no issue. So there is no such person as Marila Giannakoupoulos!
Tatiana's second marriage appears to be happier, in that she and her husband Anthony (Tony) Vamvakidis, are the proud parents of two lovely daughters, Marila and Yasmine Xenia Vamvakidis. I hope these two grow up to be healthy and happy. They will carry, however, the identical mt DNA as their collateral ancestor, the Tsar Nicholas II, unless it has mutated.
How do I know this? I am acquainted with their father, Tony, a very nice gentleman who understandably does not want his two beauties publically misidentified, at least in case of the first, Marila, as daughters of another man. This one's for you, Tony, and please pass on our best wishes from the Alexander Palace Forum to you and Tatiana and your girls. And if you wish to keep them private and not share photos, we will absolutely respect that.