GD Sergei and his spouse in the Royal mob of relatives.
The photo was taken in August 1902, in Mikhailovka, summer estate of GD Mikhail Nikolaeyvitch. The family gathered for the wedding of GDss Elena to Prince Nicolaos of Greece.

Here's my identifying of the persons in the photo:
Sitting on the ground - left to right - Prince Georgios of Greece (future Georgios II of Greece), Prince Alexander of Greece (future Alexander I of Greece), GD Dmitriy Pavlovitch, GDss Maria Pavlovna-younger, GD Sergei Mikhailovitch, GD Georgiy Mikhailovitch, Prince Andrew of Greece, GD Konstantin Konstantinovitch
First row sitting - left to right - Princess Elena of Greece (born GDss Elena Vladimirovna), GDss Maria Pavlovna-elder, GDss Elizaveta Mavrikievna (born Prss od Saxe-Altenbourg), Queen Olga of Greece (born GDss Olga Konstantinovna), GDss Anastasia of Mecklenbourg-Schwerin (born GDss Anastasia Mikhailovna), GDss Maria Georgievna (born Prss of Greece), Crownprincess Sophie of Greece.
Second row standing - left to right - Princess Alexandrine of Denmark (born Dss of Mecklenbourg-Schwerin), GD Mikhail Nikolaevitch, Princess Cecilie of Mecklenbourg-Schwerin (future Crownprincess of Germany), Prince Nicolaos of Greece, King Georgios I of Greece, GDss Elizaveta Fedorovna, GD Sergei Alexandrovicth, Crownprince Constantin of Greece (future Constantinos I of Greece), GD Vladimir Alexandrovitch, Prince Georgios of Greece, GD Dmitriy Konstantinovitch, Prince Christian of Denmark (future Christian X of Denmark), GD Friedrich Franz IV of Mecklenbourg-Schwerin, Duke Paul Friedrich of Mecklenbourg-Schwerin.