I guess i'm a little but.....
You got to let it happen.

judging by your journal you sound like an ass anyway. Let her do what she wants to

I sense a bit of competition or jealousy, but I could be wrong, so don't take it personally.

My advice is to let her learn from her own experience, and not to interfere in her life. Be there for her if she needs it, but don't butt in. Also, you said she needs to grow up before having a relationship, and while that could be true, the best way for her to grow up is to learn from experience. Having a relationship could mean gaining the experience and wisdom she needs in the future, it could make her realize that she doesn't want a relationship yet. It could end up being a positive learning experience, and you don't want to deprive her of that. You also don't want to do anything that could push her away.
WOAH THERE. I noticed in the comments that you said she was 11. That makes things slightly different. I believe 11 is a bit young for a relationship, but if it's what she wants to do, she can try it out. BUT, where she's that young, it's certainly not a bad idea for you to watch out for her. I would actually suggest it. At her age, she needs someone watching out for her. Kids these days are growing up super fast. I would keep an eye on her, but do it in a way it won't be obvious to her. You don't want her to think that you think she's stupid or something. That's how I felt sometimes when my older siblings tried to help me out, but in the end I realized they had good intentions and were just trying to help.

Out of curiosity, how old is the person she wants a relationship with? I would be super cautious if that person is older than her, that could mean bad news.

Just watch it all out on the sidelines, no need to get involved.

So.. your friend is 11 years old?

Okay.. then just play your role as the supportive big sister. Give her 'the talk' about relationships and all that jazz..
But don't try to intervene with her relationship. It's best if she learns by experience

So... you are 13yrs old... right? And if your friend is younger than you... then yeah, she's still REALLY young for love..

Ouch. There's really nothing you can tell her that will make her suddenly see reason. And nothing's really happened yet--it's just a crush. Unless she's in danger or you suspect an abusive relationship is developing, I think you should just let her learn from her own mistakes. You're certainly justified in letting her know your opinion and a brief explanation, but if she thinks you're trying to tell her what to do, it might drive the two of you apart.

I think it's too early to say that she is in love, I'd bet she likes someone, which is normal even at a younger age. And what's wrong with that? maybe you are like sisters, but you shouldn't take it personally, such things happen and age doesn't matter. I have older friends that never experienced any kind of relationship, but I have been into relationships since a young age. And besides, I don't think that her relationship will be serious, it will probably be sweet just like it is at that age, hugs and kisses, you just want to be with that person that you like, it doesn't have to mean more than that. If you don't accept this then she will probably go behind your back, and it's better that you let her know that she could come to you for advice then, even if you aren't much older than her, but still, a bit. Let her be happy, of course protect her if things go wrong, and be there for her, but let her experiment, let her experience, she is very young, she still learns, and she will learn more within time. Think of it as an interesting period, she will come and talk to you about him, and you will learn more too

Her life, her choices. Stay out of it.

How old is she?