The lady sitting next to Alexander is the president of Serbia, Maja Gojkovic.
She is not President of Sebia, but HE President/Speaker of the Parliament of Serbia.
Here is the photo of some guests:
http://www.royalfamily.org/wp-content/uploads/2017/10/NR1_0192.jpg
From left to right:
1. Sir Desmond de Silva, ex husband of Princess Katarina of Serbia, daughter of Prince Tomislav and Princess Margerita of Baden.
2. Mrs. Victoria de Silva, daughter of Princess Katarina of Serbia, and granddaughter of Prince Tomislav and Princess Margerita of Baden.
3. HRH Prince Nikola of Serbia, son of Prince Tomislav and Princess Margarita of Baden.
4. Dr. Angeliki Andrews, daughter in law of Crown Princess Katherine of Serbia.
5. David Andrews, son of of Crown Princess Katherine of Serbia.
6. Alexander Andrews, grandson of Crown Princess Katherine of Serbia.
7. HRH Princess Anne d'Orleans, Duchess of Calabria, godmohter of the groom, Prince Filip.
8. HRH Crown Princess Victoria of Sweden.
9. HE Duke Ignacio de Medina of Segorbe, stepfather of the groom, Prince Filip.
10. HRH Prince Guillaume of Luxembourg.
11. HRH Princess Maria da Gloria of Orleans-Braganca, Duchess of Segorbe, mother of the groom, Prince Filip.
12. HM Queen Sofia of Spain.
13. HRH Prince Filip of Serbia, the groom.
14. HRH Princess Dana of Serbia, the bride.
15. HRH Crown Prince Alexander of Serbia, father of the groom, Prince Filip.
16. HRH Crown Princess Katherine of Serbia, stepmother of the groom, Prince Filip.
17. HRH Princess Sibilla of Luxembourg.
18. HRH Prince Alexander of Serbia, twin brother of the groom, Prince Filip.
19. Michael, son of Allison Andrews and grandson of Crown Princess Katherine of Serbia.
20. Allison Andrews, daughter of Crown Princess Katherine of Serbia.
21. Mrs Beba Marinkovic, mother of the bride, Princess Dana.
22. Mr Cile Marinkovic, father of the bride, Princess Dana.
23. HE Countess Ana Luna de Medina of Ricla, half-sister of the groom, Prince Filip.
24. HE Countess Sol Maria de la Blanca of Ampurias, half-sister of the groom, Prince Filip.
25. Mrs. Betty Roumeliotis, the sister of Crown Princess Katherine of Serbia.