French women never die. Elizabeth, my husband's 96-year-old mother-in-law from his previous marriage, is proof of this. She is bedridden, has macular degeneration, deafness and dementia. But there is no cancer and her heart is steel. She is cared for at home in an old mining village near Lille. Marguerite has been the primary carer for eight years. She comes for two hours in the morning and four in the afternoon. Marguerite, who like me is in her mid-fifties, gets on very well with my mother-in-law. She also gets on very well with Pierre, my 72-year-old husband. Very well indeed.

Over Easter I discovered their liaison. I had guessed it when I saw them together last summer in Elizabeth's front room. Marguerite was dressed as if for a party rather than as a carer whose job was to see to an old lady's most intimate needs. When she saw me on that hot day, she looked at me with hostility. The atmosphere between my husband and her was intense. Later, when I asked him if there was something between them, Pierre denied it, telling me this was my imagination and not to be so jealous.

When he returned with me to my London flat, he phoned Marguerite daily explaining he was merely asking about his mother-in-law's health. He never called when other carers were working which seemed odd. I was sure he was lying but had no proof.

Pierre is a careless adulterer. I found a receipt for a restaurant bill on a night when he claimed he was at his French apartment watching TV. He admitted that this was “an innocent dinner” with his old mistress, Brigitte. I am Pierre's second wife. We have been married for 16 years. My husband's first spouse put up with Brigitte for 30 years. I rang Brigitte. We had spoken five years ago, when he had taken up with her again. At that time he claimed his return was merely the last spurt of a very old man.

I used to have sex with my brother

I'm not allowed to be in love with Daniel, but I have good memories of our relationship and don't feel guilty

BACKGROUND

• Sibling incest: what you think

• I hoped bulimia would make me beautiful

• Family secrets: the grief for my illicit love

• I wish I had married for money, not love

• My mother hated all women

I was so distressed that we went to marriage counselling in London. Pierre was so charming that the counsellor admitted she was attracted to him. After our sessions Pierre promised he would never see Brigitte again. Figuring I would never discover, he met her when he went back to France. There were dinners and trips to the countryside and a slide back into bed. Brigitte, like Pierre, is 72.

Now Pierre is no longer obsessed with her. Brigitte told me, with a mixture of bitterness and triumph: “Oh it's not me you need to worry about. He was certainly sleeping with me but he is madly in love with Marguerite.”

He admitted that their affair was the result of many talks in Elizabeth's house. One afternoon, Marguerite tearfully revealed that she was leaving her abusive husband. Pierre was moved and saw his chance. He asked her out to dinner and a hotel. She accepted. After sex, they both spoke of love. He told me that they decided to end their liaison after three months “because I am married to you and there was no future in this”; but in the autumn, when he learnt she had found a new lover on the internet, he became depressed.

My aged husband experienced deep rivalry when he heard of Marguerite's 60-year-old lover. He became addicted to their long phone calls. He saw her alone when he went to visit Elizabeth. Pierre made it clear that he wanted her to leave her new man for him. He told her he imagined living with her. She strung him along in case the new love affair fell through. At the same time he was superficially behaving as my loving husband.

When Pierre told me all, I asked Marguerite to leave Elizabeth's house. She refused insisting, “I have done nothing wrong. You can't make me move.” My husband refused to encourage her to go, claiming: “There is nothing more between us so why should you worry?” However, I found her presence distasteful and tried to take control.

I went to her agency to discuss her transfer. Her boss was amazed at her unprofessional behaviour but could do nothing. French employment law protects the employee, even if she has behaved unprofessionally; or perhaps this kind of inappropriate liaison is considered banal in France. I looked at her record. Marguerite had become a carer after losing her job as a secretary. In France, anyone can be a carer without qualifications. She was given no code of practice and no training. Employment law offers her so much security that she continues to receive a salary even after her charge is 6ft under.

When I learnt the truth, I broke with my husband, but now he wants me back. He declares newly found love for me, perhaps because he can no longer hope for Marguerite.

Will I stay with him? At the moment I am making no decisions. I feel betrayed. Not so much by the sexual aspect but by the constant denial of anyone else in his life. Also by the fact that he used no protection when he was with other women. My anxiety was so strong that I went for an HIV test to be sure that my husband had not infected me. Luckily the result was negative.

As for Elizabeth, she remains stable. Her appetite is healthy and her energy high. The doctor monitors any minor changes and gives her medication to ensure that her heart is protected. Each winter she gets a flu jab. She seems immortal.

I have fantasies of leaving the window open in her overheated house so that she succumbs to pneumonia. Killing her may be the only way to rid myself of this carer whose “care” has shattered my marriage. I suppose even French women must die eventually. But, if I decide to remain with Pierre, how long must I wait?

Times recommends

• Live chat: Thrifty ways to update your winter wardrobe
• Is anybody out there really listening?
• Nutrition Q&A: Will eating late make me fat?