Masterbating

Ever since my sister and i were early teens we have masterbated together . We shared a room untill we were 15 or so and thats when it begun . One night i woke with a huge hard on and started stroking it . I wanted to look at a porn mag so i turned the light on . Pretty soon my sister woke and was watching . She came and sat next to me . I had only been wanlikng for a few months and wasnt experienced . She was supprised how fat and long i was hard and wanted to touch it . I told her to slide her hand up and down real fast . I pulled up her nighty and exposed her vagina. I had seen it yesterday when she was changing but it seemed wet and the lips were open . I fingered her and she pulled me till i came . My friend at school said he does this with his sisters too . Is it normal ?