A father's uncle could be called son-in-law by his son. In some places, uncle-in-law could also be addressed as paternal uncle, paternal aunt, or paternal aunt. The origin of this form of address was because in the past, the girls of rich families were called young ladies. After they got married, their husbands had the same seniority as the girls, so the servants usually called them son-in-law. However, this form of address might vary from place to place, because differences in local customs would lead to different forms of address.
The theme of acceptance is quite common. Fem Luffy often has to be accepted by the other characters in the story. This could be because she's different due to her gender or her slightly different personality traits. Also, the theme of self - discovery is prevalent. As fem Luffy navigates the pirate world, she discovers more about herself, her abilities, and her place in the grand scheme of things.
Nami could also be a common character. She might help Luffy manage the financial aspects of his admiral duties, as well as use her navigation skills to assist in any navy - related voyages. And of course, she'd be there to keep Luffy in check when he gets too carried away.
Uncle was a respectful title for his father's younger brother. In modern terms, uncle was a form of address for one's father and younger brother. In ancient terms, the father's second brother was called uncle, and the husband's younger brother was called uncle. Uncle could also be a polite form of address for someone of the same age as his father. In short, uncle was a friendly way of addressing his father's younger brother.
In Luffy harem fanfiction, his relationships often start from friendship. Since Luffy is a friendly and open - hearted character, the female characters are first drawn to him as friends. For example, Nami might start as just a crewmate but gradually develop deeper feelings. His relationships develop as he goes through adventures together with them. They see his courage and selflessness, which makes them admire him more.
Rayleigh would be a great candidate. He's an experienced pirate and has already trained Luffy for a while. He has a lot of knowledge about the world of pirates and could give Luffy a talk about the deeper secrets of the Grand Line or how to deal with the powerful enemies that lie ahead.
One popular luffy and fem luffy fanfiction is 'Fem Luffy's Adventure'. It shows a different side of Luffy if she was a female, with unique storylines and character developments. Another one could be 'Fem Luffy in a New World' which explores how fem Luffy might interact with the other characters in the One Piece world in new and interesting ways.
I'm not sure which novel you're referring to because you didn't provide any specific information. If you can provide more context or plot, I will try my best to help you answer your questions.
Female Luffy may have a different body shape. Instead of the muscular build of the original Luffy, she could be more slender but still strong. Her hair might be styled differently too, maybe longer or in a different color. Also, her clothing choices could be more feminine while still maintaining the pirate - like style.