I can't tell you want she wants or what you should do, but she is in college and you are in High School, so it mihht not be an ideal thing.
A lot depends on how old you are ~ and, indeed, how old she is. A lot also depends on the people involved ~ their personalities, etc.
In a few years you might be fine for each other, but for now, I personally think that she is probably too old for you - especially if she is encouraging you to have sex with her, while flirting with other boys.
She probably does find you attractive, but she is going to bars and to parties for older kids. The age difference is bigger somehow in one's teens than in later years.
She feels that your brother would be against this relationship, too. Why don't you ask him what he thinks.
(There was a similar problem on the forum a while back, but then it was the younger girl wanting a relationship with a male friend of her brother.)
"The secret of success is constancy to purpose" - Benjamin Disraeli.