Of course, there are a multitude of reasons. Being a little sister myself, I think the occupation doesn’t get nearly as much credit as it deserves. As a result, I’ll admit there may be some baggage attached to this post. Be forewarned.
First, she will always play with him. Even if it’s a game she doesn’t like. Even if he never lets her win. Even if he bosses her around the whole time. She loves him and she will always play.
She tells him on a regular basis that he’s handsome. Seriously.
Whenever he is feeling low or unsure about himself I can hear him asking her, in a quiet corner of the house somewhere, “Do you think I’m good at drawing?” or “Do you think I’m good at math?”, etc. and she will always say "yes".
She gets him Band-aids when he’s hurt.
If he’s teasing her and she comes and ‘tells on him’ she quickly realizes that she doesn’t want him to actually be punished. That’s because if he gets sent to his room, she’ll lose her favorite playmate.
She gives him hugs and kisses.
She cheers for him (very loudly!) at all of his sports events.
She’s never known life without him and she’ll probably spend more time actively engaged with him than with any other person in her life.