Disclaimer: I do not own Naruto.
Acceptance.
Sasuke did not ask about their relationship, he did not talk about it to anyone and when Naruto brought the subject Itachi and Sakura, he went around regardless of whether he looked bitter or not.
Sasuke knew that Naruto had a little passion for Sakura since when they were kids, though Naruto never vocalized about it. He would never tell anyone that he shared that little passion either. He did not know when it started or why, since he always told himself that Sakura was annoying and women were troublesome and a hassle he did not need on his way to kill his big brother. At some point he decided that this ridiculous passion was an unnecessary distraction, so he pushed it deep into his mind and heart and focused on more important things, like training.
When Itachi came back and was obvious to everyone that he and Sakura had a much deeper relationship than he wanted Sakura to have with someone other than himself, he felt angry. It was anger for sure because he was not jealous. Jealousy was a ridiculous feeling that took hold of people who did not have self-confidence and Uchihas can be possessive but never jealous.
When he saw her in the woods the day they returned to Konoha after three years of absence, he felt his heart race as she took off that porcelain mask to reveal a beautiful face and eyes even greener and brighter than he remembered. When she took some time looking at him and lingered when she fixed her eyes on his long hair, he thought she was admiring him, but he discovered a little later that on that day she was just comparing him with Itachi. He was annoyed by this conclusion.
Sitting on the couch in her living room, watching her try unsuccessfully to tear a book out of his big brother's hands, it made he grunt unconsciously. When the couple stopped playing and looked at him curiously on their faces, he deviated the eyes and stood up, jumping out of the living room window and heading toward his own apartment. He repeated in his mind that it was because their play was annoying and not because he was jealous, Uchihas do not feel jealous.
He accepted with great difficulty that Sakura belonged to Itachi. When he had the conversation with his brother the day Itachi returned to Konoha and Itachi declared that Sakura was his, Sasuke understood. When an Uchiha claims something or someone, it is their and that is final. He argued that Sakura was a person and did not belong to anyone but herself, but he knew that if it were otherwise, he would attest his claim to her in the same way.
That same night, after seeing the massacre through his brother's eyes and understanding the greatness of Itachi's sacrifice for Konoha and for him, he forgave his brother completely, even though he did not say it aloud. So Sakura came out of the bathroom in those ridiculously short pajamas, making him curse his brother internally for stealing her from him without him having a chance to try, so he stroked her, knowing that was the most he would have of her and he left the apartment, leaving the two together.
Now, after a few weeks observing how much Sakura was happy beside Itachi and how much his brother was more like what he remembered when they were kids, Sasuke accepted. He knew that Itachi would never have the chance to be happy with another person because no one would be able to accept him as he was, with all the painful past and failures. He knew no one would ever trust Itachi as Sakura did.
He admitted to himself and only to himself that he would never be able to make Sakura smile as Itachi did. Then he accepted that Sakura was for Itachi and he was happy for them in a way. His older brother was home, free and redeemed, and it was more than he could ask for.
Sakura was still annoying... beautiful, gentle, captivating, strong and sexy, but annoying. And if there was anyone capable of taming that pink-haired beast, was Itachi. He was happy to be spared from work, or it was this lie that he told himself whenever he saw them together.
He jumped on the roofs of the houses and entered his own apartment, stripping off his clothes and walking toward the bathroom for a well-deserved shower. He decided that after all this fight against Akatsuki and when his brother and his teammate were married he would think about finding a woman not so problematic or with a less destructive temperament, with that last thought he decided that with a beautiful girl, delicate and definitely a civilian, he would help Itachi to revive their clan.