six (yes six!) years ago, before dh and I were together he got very very drunk one Christmas and snogged his soon to be sister in law, who told him that she was marrying the wrong brother.
He does now say that if there was one thing he could undo in his life it would be this.
It has always made me uncomfortable around her, and I know that it shouldn't for two reasons, it was before he was with me and it's an awful long time since.
And before anyone asks, yes his brother does know about it as he walked in!!