Is it normal to masterbate thinking of my sister's body?

My sister has an extremely sexy body. I've seen my mates check her out and some have even admitted to me that they want to fuck her. Now I don't blame them, she actually has a pretty face and that body is out of this world.

Sometimes I like to masterbate to photos of my sister where I crop her face out of it. She uploads a lot of outfit selfies on to instagram on a regular basis, so finding new pictures is easy for me. But i like to look at these cropped pictures of her body while I masterbate, often thinking of other girls as i jerk off, its just her body that I find myself attracted to.