Incest is so often spoken about in the same breath as abuse, but if you're close in age and equal in relationship terms then it's entirely different.
I think she was afraid to fight back.
If it bothers you later, there's therapy and other options and honestly, cross thst bridge when you reach it.
Or do the next best thing and have him catch you at it.
I think there's honestly something wrong with a brother being sexually attracted to his sister and going so far as to spy on her when she was pre-pubescent.