So, in my D&D Campaign, my character was born to the rulers of the Abifort Duchy, Duke Eristan and his wife, Guinevere, which brings me to my first question:
Next, my character (Cecilia, nicknamed "Trouble") was born a Tiefling to humans, so she was naturally hidden away (though not disowned and still has not been disowned after running away), so consider her a regular child. I need help figuring out the titles of her and her siblings. Below I will list the children from youngest to oldest, their age, and whether they are married. I ask you to tell me what their titles will be, and how someone might address them:
Your help is much appreciated.
Hey there! Since you're asking a question about a fictional universe, the "answer" as such is that it's really up to you to decide. But if you're asking because you want to emulate a particular place or time period, specifying that would help out anyone here who wants to answer, since noble titles don't really have "rules" across all of time and space. Thanks!
I can only answer for the Peerage of the UK. It is likely different in France, HRE, Italy, etc.
A duke's wife would be a duchess.
A duke is referred to as The Duke of X (or, ceremonially, as The Most High, Noble and Potent Prince His Grace [forename], Duke of X). The "the" before the title is important, which I'll come back to later.
In person, a duke is addressed as "your Grace" or "Duke".
Eldest sons of a duke use their father's most senior subsidiary title (a duke is likely also a marquess, and/or an earl, and/or a baron, etc.) as a courtesy title. They are then referred to as Marquess of X or Earl of X, etc. Note the lack of a "the" on the courtesy title; as the title is a courtesy, they are not The Marquess of X, etc.
In person they would be addressed as My Lord or Lord X. The eldest son's wife can also use the courtesy title, again without the "the".
Note: "of" is often omitted in personal address. So Duke X, or Lord X instead of Duke of X, or Baron of X.
Eldest daughters do not have courtesy titles.
A duke's other children are referred to as Lord/Lady. This is for life, but is not inherited by their children.