Re: weird dream...
You can masturbate to anything you want, it is completely harmless. Fantasies or thoughts or images in your head are private and affect no one, and no one knows. If it relieves stress/tension, if you enjoy it, it is fine.
It's typical to have a wet dream to something sexy, like the thought of your sister and her b/f having sex. Yeah, it might feel weird getting turned on by her, but it's not about her. It's about the sex, and that's arousing!