I am in my forties now and have masturbated regularly for nearly 30 years now. For the past twenty years I have wanked my weany thinking of my sister-in-law Joann. She is a couple of years younger than me and my wife, and has a body to die for.
My wife is no slouch herself and regularly watches me masturbate. I have told my wife that I sometimes jerk off thinking of her sister and it makes her very horny. I have suggested a threesome but they haven't complied as of yet.
So a couple of weeks ago I was home on vacation for the day and my wife was out shopping. I decided to jerk off in my favourite way. I set up about ten pictures around the room that I have of Joann (mostly pics in a bikini). I start jerking my meat. Everything is going great when suddenly I notice Joann at the door with a big smile on her face. I quickly cover up, but she just laughs and tells me that this is going to be fun. She then grabs my left nut and squeezes it while I am wanking. She explains to me that I am going to jerk off and she is going to make me remember this day by making my nuts sore. I agree and keep beating off for all I am worth and then cum all over myself for the first time. Joann continues her death grip on my left nut that is now in considerable pain but feels great. Joann is enjoying watching me squirm wondering if she is going to crush my balls. I soon cum again and she finally releases her grip from my left nut.
She then asks me about the pictures of her around the room. I explained that I have masturbated to pictures of her for years. She seemed amazed that I would do that but seemed to like the idea. She asked if my wife knew and I said no, I don't think she would like the idea.
Joann then grabbed my left nut again and told me she could make me do whatever she wanted. I agreed and started jerking when my wife walked into the room. We all looked at each other in shock for what seemed like eternity. Then my wife smiled and said 'so you finally got your dream come true didn't you'. I nodded and she started laughing and told her sister to come out of the room. They returned in a couple of minutes and proceeded to grab a testicle each. My wife told me that this would be a day I will not forget. The two sisters laughed and giggled for hours as they made me jerk off. This was the best day of my life so far.