I really liked 'Harry in Slytherin: A New Path'. It explores how Harry's values and actions might be different if he were in Slytherin. He has to deal with the Slytherin house's reputation and the prejudices that come with it. The author does a great job of showing how he might use his intelligence and cunning in a more Slytherin - like way.
There's this fanfic called 'Slytherin - Harry's True Home'. It's about Harry realizing that he actually belongs in Slytherin more than he thought. The story delves into his relationships with his Slytherin housemates, like Draco Malfoy. They have a more complex relationship than in the original books, as they are now in the same house and have to find a way to coexist.
One great fanfiction in this vein is 'The Slytherin Heir' where Harry, having endured abuse, is sorted into Slytherin. It delves into his complex relationships with his housemates and how he overcomes the stigma associated with Slytherin. The story shows his growth and development as he uses his Slytherin cunning to face the challenges in the wizarding world.
One great fanfic could be 'The Slytherin's Albus'. It shows Albus's journey in Slytherin, his struggles with the house's reputation and how he forms unique friendships. It's really interesting as it gives a new perspective on Albus compared to the original stories.
There's a fanfic where Harry is the heir of Slytherin and it focuses on his relationship with the Slytherin house. Instead of the usual enmity, he tries to bridge the gap between Gryffindor and Slytherin. I can't recall the exact name, but it's a really good read if you can find it. It shows Harry's growth as a character and how he uses his newfound heritage to bring about change in Hogwarts.
I'm not sure of a specific title, but generally in such fanfics, the abuse Harry faces in Slytherin could be psychological, like being ostracized or bullied verbally. Some might explore how he tries to keep his Gryffindor values while dealing with this difficult situation in a house known for its more cunning nature.
One interesting fanfiction could be 'The Slytherin Prince Harry'. In this story, Harry's sorting into Slytherin changes his whole Hogwarts experience. He has to deal with the prejudice from the Gryffindors while also trying to find his place among the Slytherins. It shows a different side of Harry, more cunning and politically aware.
One great fanfiction is 'The Ravenclaw's Path'. It delves deep into how Harry's personality would be different if he were in Ravenclaw. It shows him making friends with Ravenclaw students who value knowledge and wit, and he gets involved in solving mysteries within the castle that are more related to academic pursuits rather than just the usual Gryffindor - centered adventures.
One possible plotline could be Harry struggling to fit in at first. He's known for being a Gryffindor, so in Slytherin he'd face prejudice from both houses. Another could be him befriending Draco Malfoy early on and changing Draco's views. He might also discover a hidden side of Slytherin's history that helps him in his fight against Voldemort.
One great 'Harry Potter Slytherin sister fanfic' is 'The Slytherin Heir'. It has an interesting plot where the sister is trying to break free from the stereotypes associated with Slytherin. She forms unexpected friendships and has her own adventures within the wizarding world that are separate from Harry's main story.
In such fan fiction, Harry often faces a lot of rejection and struggles to find his place in the magical world without the support of his family.
One interesting fanfiction could be 'The Heir of Slytherin's Destiny'. It delves deep into how Harry discovers his heritage as Slytherin's heir and how it changes his relationships at Hogwarts. His friends initially have doubts, but as they see Harry struggle to use this new - found knowledge for good, they come around.