My best friend, Mo, is practically in love with my brother... she likes him a lot anyway! She is in my grade, going into 8th grade, and he is only going into 6th. Jesse, my bro, considers her a friend but sends her REALLY messed up signals. She told him to stop, but he doesn't know what is wrong with what he is doing. Today he told her not to like him anymore and she told him she won't like him. She told me, later, that she still liked him! She has a serious problem, but I still love her. She would do anything for him, but he would rather her die than himself, if you know what I mean. She I sgetting depressed and it kills me to see him torture her like that! I want to help her, but I don't know what to tell her!! Help me!

Sounds like they are both sending and having mixed signals.
Sit each of them down separate and tell them each that it is not good for them to consider each other as more than friends right now. To put their feelings on hold for when they are in high school and have a better shot at a real relationship. Right now if they were to date they could end up not liking each other and miss out for a maybe better stronger relationship later in life. Right now they are still to young and need to get in touch with who they are and what they feel and want in life better.

Learning how to interact with girls/guys is hard work, and it takes practice. And people get hurt in the learning process, it's that "hurting other people" that motivate you to do better next time.

So, let them go through this. Console them both, don't lie for either one. Butt out, but stay handy to help when asked. This stuff is supposed to be hard.

He doesn't like her, but the mixed signals he sends would be hugging her... playfully! She finds it confusing, but I know he's just being his silly self! Jesse would never date her, and we both know that! But she acts like he is the world and that he is king of his and everyone else's world!

Tell her she needs to accept him for the way he is and not read anything into it.

I've done that. SHe invited Him to Hershey with us. He went. That's how bad it is. She puts him before ALL her best friends! She needs to tell him everything and she'll let him tell her what to do. She acts really vulnerable! I always tease her about wanting a sister-in-law but she ignores me!

Why is she interested in someone that much younger? Why don't she like the boys in her grade?

She does, but not like she likes him!