How can I tell my brother that his gf likes me?

I'm 14 and my brother is 16. My brother has a girlfriend that is also 16 and I think she likes me. Ever since they have been going out, she has been staring at me. She has tried to kiss me 1 time and she pulled my bathing suit down and actually touched my junk in the pool once too. I don't even like her or find her attractive but she won't leave me alone! I feel really badly, like I'm hurting my brothers relationship. How do I tell my brother that this is happening without hurting him or their relationship? Thanks