caught jerking off by my friends sister SeXStoRY

On day I went over to my friends house. He wasn't going to be back from work for a few hours but it was on my way home, so I stopped by early to wait for him before we went out. When I arrived, it seemed like no one was home, although he was living with his s****r at the time. Since I had some free time on my hands I decided to have a jerk. I went up to the room where his computer was, which was right next to all the bedrooms upstairs. I went to my favorite porn sites, took my pants down, and started jerking away. About 10 minutes into this, I looked and caught a glimpse of his s****r in a reflection from a picture on the wall. She was standing right at the crack of the door, peaking in. I decided to not react to it, because I didn't know what else to do. So for a bit I just refocused on the porn that was before me, and continued stroking my cock. I kept glancing up at the reflection to see if she was still sanding there, and I noticed that she wasn't wearing any pants, and had her hand in her panties. It was hard to make out exactly what she was wearing in the reflection, but it looked to be just a white T-shirt and some dark colored panties. After a few minutes of looking up and back, I started to get nervous like she wasn't going to leave. I didn't know what to do since she's my friends s****r, although she's about 5'-4" skinny brunette, about C-cup tits, and a pretty face. I thought to myself if I confronted her, maybe I could fuck her. Just then I looked up and she was gone. Oh, well I thought. At least I can cum in peace now. She doesn't even have to know that I knew she was there. I can play it off like nothing ever happened. Then I notice a movement in the reflection. It's her again, but she's not wearing the white T-shit any more. I can see her nice firm tits through the reflection. She is sneaking into the room. Just as she fully enters the room, I get so excited I can feel myself about to cum. I'll never know what I did this for sure, but it was so fast I couldn't think. I stood up immediately, turned around, and there she was standing right in front of me. I started cumming immediately. I stroked out about 5-6 hot streams of cum. The first one was the most powerful, and shot up and landed between her tits, as I started to say "Oh my god". The second one shot straight onto her stomach. "What are you doing here" I said. The third also went onto her stomach. "This is so embarrasing" I continued. The next one landed right on her panties where her pussy is. She starts to laugh and say "wow that's a lot" as I pumpt the last drops onto her panties. Now I can see her in full, with my cum dripping down her body. Her tits are amazing, full and firm, with perfect sized nipples. I can see now that she's wearing purple panties, which now have streaks of white wet cum across them. She is still giggling, and she tells me she was coming in to check her email. That she didn't realize I was here. She told me she was just getting up from a nap. I know that she's lying to some extent because I saw her touching herself to me just before. Also, as she explains herself to me, she doesn't seem to be in a rush to remove my cum from her body. After I explain myself, that I didn't know she was home, and claimed to be embarrassed, (which I was to some extent, but I was so happy that I got to see her naked, and that I just jizzed on her), she wipes down and changes into normal clothes. I never had sex with her, but we did have some more fun together at later dates. We chatted until my friend got home. When he walked in he was like "oh, hey Sam. I didn't know you'd be home." She replied and said she had the day off. She never mentioned our run in.