Simon thought he knew what love meant. He loved his sister so much that he gave up everything for her. He gave up an apartment in the city, a promising career as a surgeon in Capital City, friends, family, and his fortune. He gave it all up for her because he loved her. That love drove him to risk everything and walk into a heavily armed installation and rescue his sweet mei mei.
Then they came on board Serenity and he met Kaylee. Before that moment, he never thought he'd be capable of loving anyone more than, or even as much as, his little sister. But he was wrong. Kaylee's laugh, her smile, the way her eyes sparkled when she was doing what she loved, drew him to her. He was her moth and she was his flame and he was hopelessly in love. It was a different kind of love than he felt for his sister, and that was okay. It took him a long time to realize that he had enough room in his heart for both kinds of love, but now that he did, there was no going back.
In a way it saddened him because he was experiencing an aspect of love that he could never share with River. He used to share everything with her, but that was impossible now. He loved her, but he couldn't love her the way he loved Kaylee and for a brief moment he wondered if she would ever feel that sort of love.
Then one day, he walked onto the bridge and saw her sitting there with the Captain. She sat on his lap and stared out at the stars glittering in the black. Her head rested on his shoulder and his hand was lazily stroking her hair. They didn't speak, but Simon saw the way the Captain looked down at the girl on his lap.
Simon turned and left the bridge as quietly as he entered. He wasn't worried anymore. He knew that River had someone to love her the way he loved Kaylee.