It is never a good idea to date the sister of a friend

Whichever way you look at it, when you date a friend’s sister, it is extremely likely that your friendship will suffer. Men have this understanding, this unspoken communication at primal level, and we are aware of each other’s monsters, that is why it suddenly stops being funny when your buddy and your sister get it on.
Because you know deep down there, he is an animal. There are exceptions, of course, the really good guys, but I am unfortunately not one of them.

Johnny and I were longtime friends, all the way from primary school. I knew he had a sister when we were young, but my interest in women then was like that of a goat and rice; almost nonexistent. When I met his sister years later, I could not believe that the awkward little girl I remembered had flowered into something this exquisite.
Looking at Johnny, you would never have guessed that he could have such a sister, but the evidence was right before my eyes. In short, I was smitten.

I considered getting Johnny’s opinion about what he thought of me dating his sister, but I decided against it. And him knowing some of my escapades, he would most definitely say no.
So I literally went behind his back and asked his sister out. I stole her number from his phone, pretending to be looking for something else.

When I called her, she was a bit surprised, but I have done this a number of times. I quickly used that surprise to work for me, and before she knew what was going on, she had reluctantly agreed to meet with me. Within a few short weeks, I was seeing her regularly, and things were looking good. Not only was she stunning, she had brains, and was down-to-earth. I was in this state of day dreaming about my amazing new catch when Johnny’s voice rudely interrupted.
He wanted to know what I was doing. His actual words were quite colourful, but I got the point; he was not pleased at all.

He was furious. I tried to blubber a few words, but he was not in a listening mood. He ordered me to stop immediately. I promised I would try, but we both knew I would not even bother. She was just too good to give up. He must have asked her to stop seeing me too, because next time we met, she was a bit uncomfortable.
But once I told her I was not giving up on our new found love, she brightened up and agreed we would face the storm together. And Johnny was serious about making trouble for us.

First, he told me we could no longer be friends. I knew he would miss me and come back. He did.
Then he became mean to his sister, but he is a naturally good guy, so he soon gave up on that too. What ended my relationship was when Johnny started showing interest in my young sister.
I could not tell whether he was serious or not, but it was clearly time to make a truce.