Yes, it could be. In fanfiction, there can be all kinds of creative interpretations. Maybe Jethro has a deep admiration for Merlin within the context of a particular fan - created story. It might be that the writer has crafted a scenario where Jethro is drawn to Merlin's magic, his character traits like his wisdom or his heroism.
Since this is a fanfiction, the identity of Jethro's nephew could be completely up to the writer. He could be an ordinary Muggle who has some special connection to the wizarding world through Jethro. Or he could be a character with some hidden magical abilities that even he doesn't know about until Harry Potter discovers him.
Yes, in some fanfictions, there could be a creative plot where Harry Potter is made to be Jethro's nephew. Fanfiction often takes the liberty of creating such unexpected family relationships to explore new storylines and character dynamics.
A common theme is friendship. Since Merlin and Jethro come from different backgrounds, their friendship is explored in many fanfictions. For example, they might learn to trust each other despite their differences.
Since we don't have more details, Kelly could be any woman. She might be a fictional character created for a story about birth in a city, or it could be a real person's experience that is being shared.
Well, Merlin's magic could be a big factor. If Jethro comes from a world without magic, seeing Merlin's spells and abilities might make him a fan. Also, Merlin's role as a wise advisor in many stories could attract Jethro. Maybe Jethro is in need of that kind of guidance in the fanfiction.
In most traditional 'Harry Potter' lore, there is no relation to a character named Jethro. However, in fanfiction, the author has the creative license to create any sort of relationship. So it could be possible in a particular piece of fanfiction that Harry Potter is made out to be Jethro's nephew, but this is not part of the official 'Harry Potter' universe.
One popular theme is the exploration of Jethro Merlin's backstory. Fans are often interested in how he became the character he is, so they write stories that delve into his past, his family, and his early experiences.
Kelly is the main character in this story. She is likely a person who has a weight problem, specifically weighing around 600 lbs, and the story probably focuses on her life experiences related to her weight.