Is it normal to masturbate with door open so girlfriend's sister can watch?

I leave the door cracked so my girlfriend's sister can watch me masturbate. I usually do it while I am getting ready to go into the shower (or after I shower). I wait until everyone is downstairs (except her), and then I start rubbing moisturizing cream into my cock and balls. I can see her watching from across the hall (she also leaves her door cracked, and she peaks through the crack in her door). Sometimes, when ironing clothing, I will leave my trouser zipper undone, and leave a hole big enough to expose my cock. She walks past and takes a good look. I have also left my zipper undone while we were both in the kitchen. I was making breakfast, and she came in. She noticed my cock, then left the kitchen, but came back and pretended to look for something in the cupboard. When she turned around I was facing her and she took a good look at my hardening cock.

Now I don't know if she likes it or not. What do you think, and should I continue? Anyone else do this or anything similar?