It was only my mum and I when I was growing up, so we had a fairly tight relationship, but never inappropriate.

One day when I was 19, she walked in on me jerking off to porn, we were both shocked, she closed the door and walked away. We had 'the chat' later where she told me it was natural and not to be embarrassed etc..

However after that, every now and then she would pop her head in to say something while I was rubbing one out. This happened randomly, but seemingly ever increasingly over the weeks. Each time she would open the door wider, and stay longer.
It reached a point where it felt normal to be jerking off Infront of her, sometimes she would stay a while, making small talk, others she'd just say hi and leave. The best days were when she stayed until I came, occasionally she'd make a comment like "oh it looks like you are almost done" and would pass me some tissues..
But she never touched me, only watched. I asked if she would and she always said no.

It wasn't until I was home from Uni that I got to see her, she seemingly let me catch her masturbating.. it was a bit like role reversals. I jokingly told her it was completely natural and not to be embarrassed. I sat and watched her for a few minutes untill she continued, then told her not to worry I wasn't going to try anything with her, then unzipped my trousers and began jerking myself off to her. Actually the most arousing wank I ever had.

While I was home, I very jokingly, but also quite hopefully made a 'clone-a-willy' dildo for her, and said "for the day you fancy trying".

She never did tell me how it was..