I'm 13 years old and my sister is 16. I frequently watch her when she goes to take a shower. I watch her under the door with a tiny mirror and more than once she has masturbated while inside. I then go and masturbate but feel guilty about it. is it normal to watch her and masturbate?

OK I'm 13 and I want to know if its normal to think about my girl and be sad (she's on her period)?
You could see a good bit of a girl laying down, or you could see all of them when they were standing up to get out, because of where you had to avoid the toilet.
I had many opportunities but it never occured to me, I was around 11 and my 1st cousin/best friend came over all the time during the summer. One day she was in the bath and I asked her through the door how much longer she was going to be, she said "I'm getting out now" and I don't know, I just leaned in and she had one foot coming out of the tub,drying her hair off. I saw everything.
Never got another chance with her, but during the next two years I spied on my mom quite a bit.
As far as my sister, she is a few years older than me and she use to get me to fuck her a lot, the last time she was almost 14 and I was 9. A few years later when she was married her husband would get really mad because I would go into the bathroom when she was taking a bath, like to get her kids to dress them.
I heard him say "He's probably looking at your pussy" and she would just say something like "No he's not."
Poor bastard has no idea every bit of my fingers toes tongue and dick had been in it before him.
http://isitnormal.com/story/is-it-normal-that-young-brother-get-hard-in-shower-with-old-sister-92091/
*Djboy*