1. What a woman might call her husband. The woman is from Milan and upper-class; the man is from Sicily, upper-class.
2. What a married man might call his steady, long-term mistress(es). One mistress is half-French, from Monaco; the others are Italian-American and largely first-generation. The man is the one in 1 (and yes, he's a bastard; his wife still loves him dearly).
3. What a mother might call her children, both young and older. I've got a few mothers; one has several adult children, and the others have kids varying in age from infant to teen. Mother A (adult children) is exasperated frequently with her oldest son, who is a main character; she's relatively uneducated and Sicilian. Mother B is the woman from 1, irritated with two of her sons when they fight (though she always sees the younger as her baby, even when he's grown), is highly protective of her daughter and loves her to pieces, and adores her oldest son. Mother C is the half-French one from 2, and her children are the center of her world; she spoils her son rotten and completely loves her daughter.
4. What a father might call his young children, particularly his favorite daughter. This is the man from 1. The daughter is headstrong, smart, and spoiled.
5. What a brother might call his sister from a young age on. These two are extremely close and always will be. The brother is highly intelligent; the sister is developmentally delayed. They're American-born, the children of the parents in 1, and the brother would use both English and Italian endearments. He's highly protective of and gentle with her throughout his life, even though he's an asshole to everyone else. The sister absolutely adores her brother, and he can do no wrong.
Most of these characters are bilingual; Mother A speaks only Italian, but the others speak English and Italian. The adult characters' native language is Italian, and they're more comfortable with those affectionate terms than English ones (particularly when the person they're speaking to is Italian-born). Classes are all from an American perspective, since most of the story takes place in the US.
Searched "Italian endearments"; found this post in and other general pages and forum posts, but as the linguaphiles post says, it depends on region and characters' personalities. Appending "brother", "sister", "wife", "husband", "child", and "mistress" got me largely general answers ("amore" and "tesorina", for instance) and, at one point, erotica. (The erotica was from "Italian endearments brother"; I didn't look beyond the Google snip.) I trust this community for more specifics, though. I'll probably also ask linguaphiles, but I always find this a good place to start.
I'll be gone for most of Wednesday on flights, but I'll reply to comments sometime Wednesday night.