Is it normal peep on your cousin if she want you to?

When I was 12 and my sister 14 we lived in a house with an airing cupboard which was half in the hall and half in the bathroom and had slatted doors. One day my sis had one of our cousins around for sleepover and while our cousin was in the bathroom my sister said to me come with me, she open the airing cupboard in the hall and ushered me to crawl in, she followed and we both spied on our cousin in the bath and her drying herself after. I had seen my mom and sister naked before but that seemed natural whereas this was forbidden peeping and very exciting.
Later my sister called me into the spare room where they where having the sleep over, they were both lying on the bed. Our cousin said something like 'I know you were spying on me' and I got all embarrassed. My sister then said they wanted to see me naked and came over and started to take my cloths off, I just froze but must of liked it a bit as I had an instant erection. They both laughed and our cousin told me to start playing with it, I had never really done that before so they told me what to do, it felt great. I made a mess over the floor, it was the first time I had come, and they giggled and I got dressed and left the room.
My sister told me later that she and her friends had been peeping on me in the bathroom for a couple of years and our cousin on the sleepover got excited about being peeped at and wanted me to look.
After that my cousin and myself used to wank together when we got together and my sister sometime would watch but never joined in. Has anyone else had similar experiences?