Sorry for the long story and for the fact i'm no wordsmith but here goes:
I don't know if it would destroy my life, but it would certainly fuck up mine and tear a family apart. Some background info:
I've been with my SO for almost 4 years and the event took place roughly two years ago. We were in a really bad place because I had been lying to her about things and generally being the asshole that most teen-aged boys have such a talent for being but we were trying to work things out and i had promised to never keep any secrets from her.
What happened next occurred less than a week later and I'm still not sure if I did the right thing or not. We had decided to go out drinking with her much older brother and his girlfriend, who had been together for 8/9 years and whom my SO considers a sister (we'll call them John and Katie). They had had a baby less than 2 months before and this was one of their first nights off since the baby. My SO's parents had taken the baby that night to give them a break. Naturally, we were all really hammered by the time we were heading back. The mother especially so as she still wasn't used to drinking again, which will be important later.
As we were heading back, they started arguing over something about their baby (I can't for the life of me remember what) and as drunken arguments tend to do, it blew way out of proportion. Now what I should tell you about John, which nobody knew at the time, was that he was cheating on Katie with some young assistant at his work. The alcohol, arguments and guilt didn't seem to sit to well with John, who would have guessed? So in his drunken logic he decided that rather than having a shitty dad, his baby would be better off with no dad and tried to jump in front of a moving van which luckily only clipped him and fractured his hand. Now Katie, still unaware of his reasoning, did what you would expect in these circumstances and had a breakdown so while my SO tried to talk some sense into her crying suicidal big brother I was tasked with taking her home and trying to calm her down. This was particularly tricky as she was too hammered to walk so I had to carry her as she was crying into my chest.
This next sequence of events I have kept to myself these past two years and now it is too far gone for me to ever tell anyone. As i carried this poor distraught woman home she started to change, she stopped crying and started nuzzling into my neck. I should point out that I was half her age and had just reached the dizzying height 16. Raging bag of hormones though I was, I managed to ignore the attractive woman in my arms who at this point was kissing my neck and playing with my hair. By the time we reached the house I could not have been more glad to put her on the front step thinking this uncomfortable situation was now at an end, oh how I wish.
She was so intoxicated that when I put her down, sitting her upright she immediately fell of the step and smashed her head on a plant pot. So i had to sit with her holding her up and as I was holding her she started rubbing her hand inside my thigh and trying to kiss me again. I managed to stay firm and told her it wasn't ok to do that, sadly all i managed to do is upset her. Turns out that she was being 'ignored' by John and that she no longer felt sexually attractive. So now i'm standing there, drunk as hell, stupidly thinking that telling her that i thought she was very attractive and sexy would help. As you can probably guess by now, things weren't going my way that night and she took it as a cue to relaunch the offensive and grabbed my understandably hard but unwilling dick while grinding up against me. Somehow I managed to get her to stop yet again and get her inside. By that time my SO had got her brother under control and calm and as i left the house to find them they came up the drive. We put them to bed in my SO's spare room somehow and turned in for the night.
In the morning I woke up dreading the awkwardness that was going to follow but to my relief/horror it was not to be. As I walk into the living room, there is my SO, John and Katie playing happily with their child. They all take turns thanking me for helping out and it turned out Katie didn't even remember leaving the pub, let alone the argument and her trip home with me. Looking at that little baby I realized I could never let anyone know what had happened that night when we were alone. Since then, John and Katie split up due to the aforementioned assistant fucking and Katie became an even bigger part of the family with my SO and her parents absolutely adoring her and her child. So now I'm stuck here, never telling anyone what happened, not for me but for them and all I have is an uncomfortable time every time I see Katie and a confusingly sad boner every time I remember this story.
TL;DR - The mother of my SO's nephew makes my dick hard and sad at the same time.