The dilemma My father left my mum, abruptly and with no warning, after 38 years of marriage. My mother’s sister, my aunt, recently confided in me that she never liked my dad after he, for lack of better words, made a pass at her many years ago on a family trip when I was a toddler. This has shattered me. Apart from my own sadness at having my dad’s sins confirmed, I’m extremely close to my mother and we keep no secrets. I know my aunt will never tell her the truth, but I wish she hadn’t brought me into it. I feel like I’m holding Mum back from truly moving on by keeping this from her, but I don’t want to expose her to yet more grief, and I also don’t want to deceive her.

Mariella replies It may not be news to her. As LP Hartley famously said: “The past is a foreign country,” and it’s surprising how alien it can be. It’s very unlikely that the history of your parents’ relationship since they first embraced is clearly divided into right and wrong, or the guilty and the blameless.

When someone departs after 38 years of marriage, the new life they embark on can come as less of a surprise than the secrets buried in their old one. When a couple crumbles, their long-shared intimacy is given a makeover. The entire relationship, from start to conclusion, is liable to be reshaped in the sad but seemingly inevitable propaganda war and apportioning of blame that follows the end of love.

Your aunt may have carried this secret with her for decades, or refashioned it to suit today’s more favourable environment, but either way, dumping it on your shoulders seems unfair. If she felt compelled to open her heart to anyone, it should have been her sister some time ago, not when the emotional harm is starting to heal. Using you as a go-between with a piece of unsubstantiated anecdote isn’t acceptable.

Learning about this small betrayal is not the key to your mother moving forward, even if it is rooted in hard facts

You can be sure of one thing and that is that this small betrayal is not the key to your mother moving forward – even if it is rooted in hard facts. The idea that she’ll leap for joy the minute she hears that your father tried to seduce her sister is really not credible. It would be most surprising if, over the decades, there hadn’t been transgressions, allowances made, blind eyes turned, to avoid what was easier not to see. I’m not talking specifically about adultery or condoning it and I’m certainly not absolving your father of his possible misdemeanours, just pointing out that in many ways being fed a variety of viewpoints and being left to work out if they were accurate, is worse than knowing.

Accepting that no union is perfect means overlooking small defects to focus on the bigger picture – and that is what happens in most long-term relationships. It’s perfectly possible that your father was a flirt and he may indeed have been a serial philanderer, but your aunt’s description of this past attempt at seduction is definitely not conclusive proof of either. I suspect your mum will have been more aware of your father’s failings than she shared with you and that her way of coping involved a degree of wilful blindness.

We may not like it, but we all have a flexible relationship to “truth”, defining it as a literal, tangible thing when it suits us and when it doesn’t we expand the parameters to allow plenty of wriggle room. In situations that don’t involve an outright lie, veracity gives way to stories that shift and change depending on our mood, our relationships and our interpretation of the actions of others. Your aunt has placed a weight on your shoulders, but context is important, too, and peering into deep jars when you can’t see the bottom reveals very little.

I’m sorry to hear about your father’s departure and particularly the abrupt and inexplicable nature of it. You should definitely try to have an honest conversation with him about that. Meanwhile it’s great that you are supporting your mother, but she needs to stand on her own two feet. You are her child, not her emotional support system. I’ve no doubt you are being torn in two in terms of your loyalties and I can only advise from my own experience that you should fight to stay as neutral as possible. Love is irrational, from elated beginning to bitter end, and you are not there to sit in judgment on your parents’ choices.

No matter what divisions exist between them, they remain your parents and you have the right to maintain healthy relationships with them both, not suffer lobbying from either side of the fence. Your aunt’s admission may have thrown light into the darkness or represent another twist in a tangled tale. Either way you need feel neither guilt nor responsibility, just sadness that the love that made you didn’t last the course.

If you have a dilemma, send a brief email to mariella.frostrup@observer.co.uk. Follow her on Twitter @mariellaf1