At first, Naruto would likely be extremely confused. He would try to find reasons for Kushina's actions. He might start to question his own worth, thinking that he has done something wrong. As time passes, he could become angry. Angry at Kushina for leaving him and at his sister for seemingly taking his mother away. This anger could fuel his determination to prove himself and show Kushina that she made a mistake.
Naruto might initially be in shock. He has always counted on Kushina's love, and this sudden abandonment would be a huge blow. He could become withdrawn and quiet, not understanding why his mother would do such a thing.
Naruto would be devastated. He sees Kakashi as a mentor and father figure. He might initially go into denial, not believing that Kakashi would actually abandon him.
At first, Naruto is likely to be heartbroken. Konoha has been his home, and he has always longed for acceptance from the villagers. So being abandoned would be a huge blow to him.
He could become very determined to find out why Sakura left and try to win her back. Naruto is known for his perseverance, and he wouldn't give up easily on his family. He might also start to question his own actions and wonder if he could have done something differently to prevent this.
He may develop trust issues. Being abandoned by his mother could make him more cautious about getting close to people. However, this could also make him more determined to prove himself. He might strive to be the best ninja in the village to show that he is worthy of love and acceptance, despite being abandoned. For example, he could push himself extra hard during training.
At first, Naruto might be extremely confused. He has always considered the village his home and Kushina as someone important. So being banished by her would leave him feeling lost and not understanding why it happened.
In the fanfiction, it could be that Kushina is pressured by external forces. For example, there might be a powerful group or individual who forces her to abandon Naruto for his sister. This could be part of a larger political or power - play scheme. Also, it's possible that Kushina has a moment of extreme emotional confusion. She might see something in Naruto's sister that she once desired for herself when she was younger, and in a moment of weakness, she decides to focus on the sister instead of Naruto.
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.
Naruto might be extremely heartbroken. He has always looked up to his mother, and being disowned by her would be a huge blow. He could become withdrawn and start to question his own worth.
Well, it could go in many directions. For example, his family might start to search for him, which could lead to a series of adventures where they encounter new characters and face various challenges. His wife, if he has one in the story, might become stronger and more independent in his absence, vowing to bring him back.
Kushina would be extremely worried. She would rush to Naruto's side immediately and start using her medical knowledge or any available resources to help him get better.