Is it normal to have sex with your grandma?

Since I'm 18 I am always horny and I like old people. My Grandmother was always like my second mom.She always helps me out in my problems. And I think she kinda likes me after there divorce with my grandpa. Sometimes she sleeps in our house in my room. But since then every night I feel something touching my penis. I thought at first it was a dream but no. I opened my eyes a little bit and my grandmother was.... how should I say this?! EXERCISING my penis.