[M] My GF Sister: Take a Risk

I've been fascinated by my friend's sister for some time now. She's been training at the gym for a few months, and the results are quite noticeable. She's a bit shorter than me, I'd guess around 1,84 meters, has a heart-shaped ass, a slim waist, and smaller breasts – exactly my type.

Now, to my story: We went jogging together one day, and I pulled out my phone and started filming her. The slow-motion clips turned out great, especially when her ass swayed seductively. Over the following weeks, I took more and more pictures of her and jerked off to them in the evenings. Now, I want to take it a step further and stroke my cock while she sits across from me and can watch, of course, discreetly. She noticed, but didn't do anything to look away. It almost seemed like she enjoyed watching me. What do you think about this? What should I do next?