Right when Sakura was born, she knew she wasn't alone. A small voice inside her head liked to talk to her when things seemed dull or when she needed to be cheered liked to call this voice Inner.
She never told anyone else, in fear that they would call her names or try to take Inner away. So, she managed to only talk to Inner within her head rather than outloud. When she was younger, people thought Inner was her imaginary friend, so there was no red flags raised. But as she grew older, more people began to point out her defect of talking to 'herself' too much. So Sakura hid it from view or hearing.
Inner was like the sibling she never had.
They had knowledge beyond her comprehension and helped her sky rocket through school with top grades. They gave her advice on how to dress and do with her hair that would please Sasuke or get Ino jealous and they even helped her stand up for herself, whispering words of encouragement and hateful things to say to the bully.
When she grew up more, it became plainly obvious that something was wrong. Normal ninja shouldn't have voices in their head like this. So she looked into it, without her parent's knowing of course. She didn't want them getting in the middle of her shenanigans.
Checking the hospital records, she found out that her mother had known she was carrying two children. Baffled by the sheer fact that her mother didn't tell her, she confronted the doctor who had birthed and took care of Mebuki. He said the second baby inside Mebuki couldn't sustain enough vitamins and growing material considering two reasons Sakura was in there and her mother couldn't provide enough for two. The baby had died in the 22 week pregnancy period and was removed shortly after Mebuki had given birth to Sakura. They didn't want to risk hurting Sakura with removing the baby, so they waited.
Knowing this, Sakura returned home. But she didn't confront her parents, rather she had a long length conversation with Inner. In a way, Inner really was the sibling she never had.