It could also be that the Potters are under some sort of magical influence or curse. In the fan fiction world, there could be a dark force at play that makes them see the twins as more important. This force might be manipulating their emotions and perception, leading to the neglect of Harry.
One possible reason could be that the twins might have some special abilities or circumstances that draw the Potters' attention more. Maybe they are more outgoing and seem to need more immediate care or guidance. Another reason could be that there is some sort of miscommunication or misunderstanding within the family. For example, the Potters might think Harry is more self - sufficient when in fact he also needs their love and attention.
Harry might become withdrawn and introverted. Feeling unloved and unwanted, he could start to keep to himself more, spending time alone in his room or exploring the grounds of their home alone, trying to make sense of why his parents seem to favor the twins.
He might become withdrawn. Feeling unloved and ignored, Harry could start to keep to himself more, not sharing his thoughts and feelings with his family. He may also become resentful towards the twins, seeing them as the cause of his being neglected, even if it's not entirely their fault.
He found a novel about the twin brother. This novel was called " Loving You Just Accidentally ". The male protagonist was a fraternal brother and younger brother. The female protagonist first met her younger brother, then her older brother. Then, she fell into a relationship with her older brother, unable to extricate herself. At the same time, she struggled with her feelings for her younger brother. This novel might meet your needs. I hope you will like it.
Sirius Black could also be part of it. He was Harry's godfather. He might have escaped from Azkaban earlier in this fanfic. He teams up with the Potters to rescue Harry. He could be the one who scouts out the Dursleys' house before the actual rescue operation, using his skills as an Animagus to stay hidden.
Perhaps there's a plot against Harry by some dark forces, and the Potters believe that only they can protect him properly. They might think that the people who have been taking care of Harry so far are not equipped to handle the new and more dangerous threats. Also, it could be that the afterlife in the wizarding world has some rules or opportunities that allow them to try and get Harry back into their family fold. They could have been given a second chance, so to speak, and are eager to take it.
One possible motive could be that the Potters discovered some dark plot by the Malfoys involving Harry. Maybe they found out that the Malfoys were planning to indoctrinate Harry into their pure - blood supremacist ideology against his will. So, to protect Harry from that, they kidnapped him.
The twin could change the plot by being more interested in academic magic than Harry. This might lead to him uncovering secrets in the library that Harry misses, which could be crucial in the fight against Voldemort.
One common theme is loyalty. The twin brother is always loyal to Harry, standing by him no matter what. For example, in many of these fanfics, when Harry is wrongly accused, the twin brother is the first to defend him.