My sister is a bright graduate who has always been stubborn and single-minded and needed intense, passionate relationships. When she met her husband and they married and had kids, I assumed she had at last found happiness. But for the past two years, she has embarked on an affair with someone she met at work.

She is being very open about the affair, with our family and with her husband. She says the other man needs her and she needs the romance of it as marriage isn’t enough. He comes to their house and she expects her husband to socialise with him. My brother-in-law has agonised about the situation but doesn’t want to leave his house and family. He expects, as we all do, that eventually, it will fizzle out. But it hurts him.

I am starting to worry for the children, who are aware of Mum having this “friend” (we found out from the kids that she recently took them away for a weekend with the other man) and starting to get old enough to realise something is going on, and also relationships between my sister and our wider family. My parents have tried but failed to talk to her about it and she is impossible to talk around on any subject, consistently of the view she is right and we are wrong. Her argument is that because her husband is “OK” with this, we should all be too. To keep positive relations with her, my parents are being asked not only not to judge but also to accept what she is doing, which I consider unreasonable.

She has always had this stubborn character, but about five years ago self-diagnosed herself with a condition [details not published]. When you read about women with this condition, it does help to understand her. She has refused to seek any professional advice for it, preferring to do her own reading. But she is bright and I feel, on some level, she must know that the situation is hurting her husband. I haven’t got involved so far because I know she will simply shut down communication once we try to discuss it.

Is there any way we could get through to her the damage she is doing to those around her for the sake of her own happiness? Or do I simply sit on the sidelines, supporting her husband and my parents and waiting for the inevitable fallout?

**********************************************************************

Open or polygamous marriages are the way some people operate their relationships, of course, but it does need both sides to agree. And it doesn’t sound as if this is happening here.

Although I think affairs between a couple are private, this isn’t really an affair she is conducting in private. She is asking you all to, in effect, join in – to accept another partner in her life when, it seems, her own partner does not. You are right that you cannot stop her, but she cannot make you approve of the situation if you do not.

I contacted the psychotherapist Julia Cole (bacp.co.uk), author of After the Affair, who thought what was “interesting is that, usually when people have affairs, there is a ‘bubble’ effect, which protects them from what’s happening in the real world. But that isn’t happening here.

“What I think is happening here is that this is like a defence mechanism. Your sister is not having to deal with issues with her husband. My feeling is that this [the affair] is some sort of smoke screen for your sister, diverting her from something else that’s going on. I would like to think whatever she’s diverting from should be the focus.”

Any idea what that might be?

We both felt that the self-diagnosis of a condition was a red herring, as Cole puts it. And no justification for her behaviour.

I feel she isn’t taking responsibility for her behaviour at all – is she usually like this? Or is she absolving herself, now, of responsibility because she is usually the responsible, sensible one? There is something about her behaviour, as if she is saying: “I’m different and I don’t have to live by normal rules.” I feel there is something very gauntlet-throwing about her behaviour, almost as if it were a cry for help.

Of course, your primary concern seems to be for her children. I asked Cole how she thinks they may be feeling. “Quite confused – they may start asking difficult questions soon. They may even start regressing.”

What to do? Cole recommends, as your sister seems to have quite black-and-white thinking, asking her “very specific questions, such as ‘How do you think this is affecting your husband/your children?’ Not in a guilt-trip way but in the spirit of inquiry. Some people just don’t understand how they can affect others.”

Cole advises that if your sister is just defensive or dismissive, you could gently press further by saying something like: “Have you noticed your husband is much more sad than usual?” If she says “no”, you could then try saying something like: “I’ve noticed it. I wonder why you haven’t?”

If your sister really will not take responsibility that her behaviour is hurting her family, all you can do is offer them support – but try making it clear you don’t take sides and you are there if she wants to talk. “You could also,” says Cole, “encourage your brother-in-law to get counselling on his own.”

The pertinent question I would ask your sister is: “How would you feel if your husband were doing this?”

Your problems solved

Contact Annalisa Barbieri, The Guardian, Kings Place, 90 York Way, London N1 9GU, or email annalisa.barbieri@mac.com. Annalisa regrets she cannot enter into personal correspondence.