Well... There are many issues creating an order of succession. First, Charles eldest is only 18 or so by the times Carine the 5th Princess is born and in the same age group and Lelouch, Marrybel, Euphie, and Nunnally. Second, Apparently there are numbered princes up to 36 at least, although the highest numbered princess is only 5, several major princesses have no number. Third, due to the limited time between Odysseus and Lelouch's birth, Charles had apparently 17 people in line for the throne, but only had 11 Princes... and 2 Princesses. There can't really be grandkids grabbing spots because again, Odysseus was unattached, same with Schneizel and Clovis, and those are all born before 1995. There's not enough time for some newborns to take up the spaces in succession
There's a few way to explain away some of these but ultimately there isn't ONE solution for making this work. If I follow birthdates strictly, At least 4 sons of Charles died before Lelouch was born, but kept their place in succession but not numbered title for some reason. If I go by the numbered prince system indicating a birth order, then it's explainable... except for Euphie's born AFTER lelouch and is the 3rd Princess. Oddly this would all be so much simpler if Euphie was like the 6th or 8th Princess. But that would make things explainable. The other option is that I adjust some people's birthdates, Euphie's and Carine's to make it seem like Charles isn't killing daughters behind the scenes.
Oddly there is one way to explain this. The numbered titles indicate birth order separately between Male and Female. The ranking in the order of succession is the monthly "rankings" so to speak that each heir gets in order of precedence. Odysseus is always first after the Emperor and his Empresses, since he is first in line. Same goes with Guinevere. Schneizel is well SCHNEIZEL, Cornelia is a leading general of the Brittanian military, Clovis is dedicated to R&D and reconstruction forces. They are the eldest and have had the most chance to prove their worth. Charles just has his butlers monitor everyone else's accomplishments and ranks accordingly. Lelouch as an 8 year old kid via his connections managed to be ranked 17th above how many siblings and leading nobles is a remarkable feat... Which he threw away. Below is a hypothetical way to do it. Names and middle... initial I guess? with stars indicates the person has no bearing on canon.
Most of the names below that don't have articles are really just place holders, and various appropriate sounding middle names to be Empressess
The Royal Issue of his Imperial Majesty Charles zi Brittania
1st Prince:
Odysseus eu (1984)
1st Princess:
Guinevere de (1986)
2nd Prince:
Schneizel el (1988)
2nd Princess:
Cornelia li (1990)
3rd Prince:
Clovis la (1993)
4th Prince: Alexander* xe*
5th Prince: Agamemnon* eu*
3rd Princess:
Euphemia li (1999)
6th Prince: Joachim* von*
4th Princess: Katherine* du*
7th Prince: Cecil* eu*
8th Prince: Sven* ex*
9th Prince: Bismarck* el*
5th Princess:
Carine ne (1999)
6th Princess: Elsa* ex*
10th Prince: Lancelot* de*
7th Princess:
Marrybel mel (1999)
11th Prince:
Lelouch vi (1999)
8th Princess: Katryna* ne
12th Prince: Claudius* di*
13th Prince: Sergio* cei*
14th Prince: Peter* ni*
9th Princess: Anna* ex*
10th Princess: Laila la
11th Princess: Hanna* von*
12th Princess: Lillybel* rui*
15th Prince: Hadrian* ex*
16th Prince: August* bi*
17th Prince:
18th Prince:
19th Prince:
13th Princess:
Nunnally vi (2003)
14th Princess: Locusta* di*
15th Princess: Miranda* di*
16th Princess: Natalya* di*
17th Prince:
17th Princess:
18th Princess:
18th Prince:
19th Prince:
19th Princess:
20th Prince:
20th Princess: Julia mel
21st Prince:
21st Princess:
22nd Prince:
22nd Princess:
23rd Prince:
23rd Princess:
24th Prince:
24th Princess:
25th Prince:
25th Princess:
The Empresses of the his Imperial Majesty Charles zi Brittania
Note: Only consorts that bear children are given the title Empress
1st Empress: Anticlea eu Brittania
2nd Empress: el
3rd Empress: Victoria li Brittania
4th Empress: Gabrielle la Brittania
5th Empress: ne
6th Empress: xe
7th Empress: Marianne vi Brittania
8th Empress: von
9th Empress: Flora mel Brittania
10th Empress: ex
11th Empress: rui
12th Empress: da
13th Empress: di
14th Empress: cei
15th Empress: bi
16th Empress: de
17th Empress: kei
18th Empress: san
19th Empress: qua
20th Empress: je
21st Empress: ni
22nd Empress: hy
23rd Empress: gua
de and du are not Empresses as they accepted larger stipends from the royal treasury rather than the title of Empress. However they are still Royal Consorts
Again I welcome any wanted additions or worldbuilding discussions about Brittanian society or history. Discussion makes the thread better.