I secretly want my younger sister
Why do you want her?
Because I'm attracted to her.
It's gross you want to be romantically involved with your sister. You need to seek professional help.
My sister is so beautiful, she has a better body than my friend and she is really smart.
That doesn't matter because she is your sister and being romantically involved with her is incest. Plus, if she's younger than you then this also counts as pedophilia which will land you in prison.