They had never noticed the void that was left in themselves until it was filled by her presence.
Never, since they had been separated, did they think they were less than whole. Sure, there was the loneliness of solitude and the exclusion they had from one-another due to the separate custody of each brother, but that was a passive thing, something that was always there. They knew of that emptiness inside themselves, had grown around it and filled it with hope and wild dreams of eventually having their broken family put back together, or at least four of five pieces.
Sakura was probably fine where she was. She wouldn't remember them, that girl was too young when they had been forced in different directions. Her life was probably better without four crazy brothers to weigh her down, so they never thought about it.
But then her grandmother died, and they were forced to join her. Perhaps 'forced' is not the word to use, for they jumped on the opportunity without a second thought of her previously-normal life or the consequences and hardships that would face them due to their one-minded decision as a single cohesive brother-unit.
Once she was in their life, though, she infected them. All the memories, all the feelings of their darling little sister came rushing back like a river into the ocean, the water cycle taking parts of that great body of water and transporting them all over the world so they would need to make that journey back to its homeland; the sea. She spread throughout their mind, poisoned their thoughts until her livelihood was all that mattered, her happiness the greatest thing to preserve in all of life. Sakura entered their hearts, found that hole that hope and dreams filled and put something more filling inside, put warmth and happiness and those darling little smiles in that hollow hole.
She was in their life, she was in their beings so deeply it would be impossible to get out.
But that was fine – they didn't want her out, anyways.