I was delighted when my parents announced she was coming for Christmas. As an unsophisticated 14-year-old, I thought she would add to the festive air about the place. I felt such excitement before she arrived: this funny lady who was going to make Christmas such a riot.
It had only been a few weeks earlier that my mother had phoned from my grandmother's to say that she had met an old school friend and was bringing her home for the weekend.
It never occurred to me that it was a little short of amazing that she had run into Sue in the remote part of Norfolk where my grandmother lived. Nor that she was able to drop everything to come 100-odd miles out of her way at a moment's notice.
Even when they arrived and seemed a little vague about what they had done together as children, no alarm bells rang. Life was straightforward. Most of all, I had no reason to doubt what my parents told me.
On Christmas Eve, I went to Paddington station with my father to meet Sue and felt very grown up to be going with him, instead of with my mother. He seemed excited and jolly - I did wonder why, considering that he hardly knew her. I was slightly anxious that the conversation between them in the car going home might be stilted.
When Sue arrived on the platform, they kissed briefly. There was an air of familiarity between them; I noticed it but almost immediately put it out of my mind. I just wanted to get back to start our family Christmas.
And, for me, it was a lovely one - probably, the last really innocent Christmas I had. If Sue was the centre of attention, then as a guest that was only right. If my mother seemed at times overwrought, who could be surprised with all those mouths to feed? What other reason did she have for snapping at my grandfather when he urged her to go the pub for a Christmas drink with my father and Sue?
Over Christmas lunch, it emerged that Sue was getting over an unhappy relationship; she had decided she couldn't marry her boyfriend because she didn't love him. How brave she was to turn down marriage, I remember thinking, especially as she was getting quite old, almost 30. How lucky she had such a nice family with whom to spend Christmas.
Then, as everyone was laughing and joking, Sue took the rather ancient silver saltcellar and shook it over her food. The top flew off and the contents landed on her turkey. Her food was ruined and everyone was aghast. There was silence; the mood changed instantly. I couldn't understand why my mother seemed quite so upset. It was only much later that she told me why the incident nearly brought her, emotionally, to her knees. She was sure Sue must have thought her distress was deliberate. The delicate edifice of civilised deceit looked like crumbling. Later that day I heard Sue crying in the bathroom. My mother went in to see what was wrong. When she came out, she said Sue was upset about her boyfriend. Recalling the lunchtime conversation, I was puzzled but decided that, like most things with grown-ups, it was terribly complicated. Years later, I discovered that Sue was in tears because my father was watching television and ignoring her.
It was this as much as the other poignant memories of that Christmas that are most vivid now - catching the odd expression in my mother's eyes as I lay on the floor, my head in Sue's lap as she talked and stroked my hair; the silence when I asked if Sue could come next year and my younger sister's inexplicable dislike of her. She had seen and heard things I had missed, and never fell for the tissue of lies.
The idea of introducing Sue to my sister and me had been my mother's. She had known about the affair almost as soon as it had begun because my father had told her. But they carried on living together for two years while the affair went on.
Finally, my mother realised that he wouldn't end the affair and would probably leave. She was still in love with my father but thought that we should meet his mistress so that when he left we would already know Sue and would feel less alienated from him. After I was told the truth of that Christmas, I thought my mother had behaved heroically, even angelically.
Now I am not so sure. I feel the charade disabled my anger at my parents' separation. I couldn't be angry with my mother; nor with my father because I was terrified of driving him away. Worst of all, I couldn't be angry with Sue because, by the time I knew the truth, I liked her. My anger was turned inwards and led to years of periodic depression.
Now, 15 years on, I still recall that happy Christmas, before my innocence was lost, before my father left for good. Three years later we all got together again - my father and Sue, now my stepmother, and my mother and her new husband. But this time it was my turn to play Christmas charades.
The author wishes to remain anonymous.