Royal Family of Diana
Diana has a very simple rule to become crown prince. Kill all of the other royal family members who oppose you and come back with a stamp from each of the other siblings who weren't killed and are on your side.
You can not kill a royal family member if they already decided upon the title of Duke. A duke cannot usurp the crown for power. That is a death penalty.
In other words, stamp the paper or die.
In Diana, there are ten royal children
The Line of succession is as follows
First Prince Terence
First Princess Freya
Second Prince Paris
Second Princess Lilith
Third Prince Michael
Fourth Prince Edward
Fifth Prince Albert
Sixth Prince Hades
Seventh Prince Zachary
Third Princess Iyla
Gender does not matter when you are an heir. Though it works differently in the nobility, the rule lies strictly. You shall not be discriminated against because of your gender.
So then here's a quick profile!
Terence
Age: 29
Status: Engaged
Mother: Empress Alexandra of Diana
Freya
Age: 29
Status: Single
Mother: Empress Alexandra of Diana
Paris
Age: 27
Status: Married to Princess Elena
Mother: Second Consort Aurora
Lilith
Age: 26
Status: Divorced
Mother: Third Consort Phoena
Michael
Age: 24
Status: Unmarried
Mother: Fourth Consort Freesia
Edward
Age: 24
Status: married to Princess Cyra
Mother: Fourth Consort Freesia
Albert
Age: 22
Status: Married
Mother: Third Consort Phoena
Hades
Age: 21
Status: Studying
Mother: Fifth Consort Arietta
Zachary
Age: 18
Status: Single
Mother: Fifth Consort Arietta
Iyla
Age: 8
Status: Single
Mother: Empress Alexandra of Diana
As of right now there are four consorts including the empress. Third Consort Phoena died while giving birth to Albert.
Each prince and princess takes their mother's last name. Their last name changes when they officially become crown prince or crown princess.
This is to avoid conflict with the crown prince/princess.
In the case than an heir is unable to be produced, they can adopt from any of the dukedom families. This has never happened before though.
The Empress does not have to be noble birth. She only has to be a capable ruler and mutually in love with the emperor.