We all want a shining knight to come to our rescue and feel like a princess, when we are young.
You're not sexually active but at the age when you are starting to feel like a "woman" that's natural and normal.
You are at the age where you want your knight to come and rescue you, and have a loving relationship that is real.
So, your dreams reflect "someone" saving you and the only one that has is your Brother and so, he comes into the dreams.
I suggest you visualise someone that you have a crush on, a movie star, singer and start visualising that person as the one that saves you.
On a phycological point of view, it's not healthy seeing your brother in these dreams in that way.
On another note. Not to be rude. But, he is older than you and protects you, think back as to whether he has ever been in-appropriate that you don't remember, can't recall. And, if not him, an older man, a Father figure. Sometimes we supress things.. Hypnotherapy may help there but have a deep honest think.. Because, that could be the only other explanation in "my opinion" as to why you are having those dreams.