One possible plot could be that Lemon and Harry discover some long - hidden family secrets together. Maybe they find out about a special power that runs in their family that Voldemort is also interested in. So they have to race against time to master this power and keep it out of Voldemort's hands.
Ron and Hermione are likely to interact with them. Ron might be a bit more protective of the sister, while Hermione would probably study and share knowledge with her. They could all be in the Gryffindor common room, discussing spells and strategies.
One possible interesting plot could be their adventures at Hogwarts together. Since they are twins, they might have a special bond that helps them face challenges like the Triwizard Tournament. They could work as a team to solve the mysteries surrounding it, using their combined magical skills and the unique connection they have as twins.
Well, obviously Harry Potter is one of the main characters. His twin sister would be another main character. There might also be the usual suspects from the original series like Hermione and Ron, who would likely be involved in the story as friends and comrades to Harry and his sister. Dumbledore would probably also play an important role as he does in the original books, guiding and protecting the young wizards.
One common theme is family. Since she's Harry's twin sister, the stories often explore their family connection and how it affects their lives in the wizarding world. Another theme is identity. The sister has to figure out who she is in relation to Harry and also as an individual witch. Magic is also a big theme, of course, like how her magic might be different or similar to Harry's.
Another way their magical abilities could differ is in the area of spell - casting. Harry is good at defensive spells, but his sister could be excellent at transfiguration. This would mean that in different situations, they would rely on each other's skills. For example, when they need to transform something to solve a problem, his sister would take the lead, and when they are under attack, Harry would be the one to defend them using his defensive magic.
One interesting plotline could be the twin sister using her unique bond with Harry to help him in his battles against Voldemort in ways that others can't. For example, she might have a special magical ability that is only enhanced by their twin connection.
One popular 'Harry Potter Twin Sister' fanfiction is 'The Lost Twin'. It has an interesting plot where the twin sister discovers her hidden powers. Another one could be 'A Sister's Bond' which focuses on the relationship between the twin sister and Harry. There's also 'Twin Secrets' that delves into the secrets the sister has been keeping.
Often, there is a theme of discovery. Maybe they discover new magical abilities together or find out something about their family history that was previously unknown. Also, the theme of protection might be present. Harry might feel a strong need to protect his twin sister, or vice versa, especially in a dangerous magical world full of dark forces.
Hermione would still be a very popular character. She could be the one to help the twin sister study and understand the more complex aspects of magic. Her logical thinking and vast knowledge of magic would be invaluable in their adventures.