Naruto would probably be overjoyed at first. After all, he has always longed for his parents. But with the sister, it might be a bit more complicated. He might be a bit cautious, not sure how to interact with her at first. But eventually, he would welcome her into his life with open arms.
One possible storyline could be that Naruto's parents' return brings a sense of closure to Naruto. His sister, perhaps, helps him adjust to this new family dynamic. They could all go on adventures together, with Naruto showing his family the village he loves so much. His sister might also have some unique abilities that add to their family's strength.
Some of the more hot - headed ANBU members might overreact and assume Naruto is a threat to the village. They could try to use force to get the stolen item back. However, the more level - headed ones might try to figure out Naruto's motives first. They might consider that Naruto could have a good reason for what he did, like if he thought the item was being misused by the ANBU.
Naruto's relationship with Sasuke could become more complex. With his family back, Naruto might have less time for Sasuke at first, which could make Sasuke feel jealous or left out. But on the other hand, Naruto's newfound family support could also make him more confident in reaching out to Sasuke. He could try to get Sasuke to see his family as a positive example of what he could have too if he let go of his revenge.
The Anbu sister could be a powerful and mysterious character. In the fanfiction, she might have unique abilities that Naruto is curious about. Their meetings could be filled with tension at first, as the Anbu are often secretive. But as Naruto's friendly nature breaks through, they start to share their stories.
In fanfiction, the teen Anbu sister might be from a different clan. Her relationship with Naruto could develop as they face common enemies. For example, they could team up against a group of rogue ninjas. She could teach Naruto some new Anbu - like techniques that he hadn't known before, which would enhance his skills and make him more formidable in future battles.
He may be surprised. Fanfictions can have some really out - there ideas that he might not have considered before. For instance, a fanfiction might have Naruto suddenly having a new, super - powerful jutsu that was never seen in the original series.
Well, one possible plot development could be that Naruto's parents and sister help him train even harder. His father, Minato, might teach him some new and powerful jutsu. His mother, Kushina, could provide emotional support and also share some Uzumaki clan secrets. And his sister could be a new friend and ally for Naruto in the village, perhaps even having some unique abilities of her own that they can use together in battles or to solve problems in the village.
She could be very sad. After all, Naruto is likely an important person in her life. She might feel abandoned and start to question why Naruto left her.
Their return would likely have a huge impact on Naruto. He would finally have the family he always longed for. He might become more confident and less of an outcast as having parents is a normal thing in Konoha. He could also learn more about his family's history and jutsu directly from them.
One possible fanfiction could be about Naruto forming an unexpected bond with Kurenai's sister who is in the Anbu. Maybe Naruto stumbles upon her during a mission and they start to work together, gradually learning about each other's pasts and dreams.