My friend created a peephole to spy on his brother, and so for years I started using it to peek on his sister while she showered.
Why did you watch your friend's sister taking a shower for years?
I couldn't help myself because she was so attractive
You are breaking the law by spying on someone while they are taking a shower. You behavior is repulsive and it needs to stop immediately. Get help for your voyeuristic tendencies.
It's fine, I liked it and she is even more attractive now that she's older and that I know her better.
People would think that you're a creep if you continued to peep on this woman, and it is extremely wrong to ever do that. You shouldn't spy on people, even in an non-sexual manner.