The dilemma I have been with my husband for 13 years, married for 11. We have children but not together. My daughter was five when we met, she is now 18 and my husband’s only child is 22. He recently moved back home with us and he and my daughter formed a very close relationship with each other. Back in July 2017, it came to light that my stepson and my daughter were having a sexual relationship. This has been going on for six months now and I recently found out that she is pregnant. I have seen my daughter for a total of an hour in the past five months as she moved out with my stepson to his mother’s. We have tried to talk on the phone, but it never ends well. I know some people feel it’s OK because they are not blood related, but they were raised as family and my husband and I feel betrayed and our family circle is broken. I miss my daughter like crazy, but I worry that the more I try the more damage is being caused. I want us to be a part of each other’s lives, but I am too hurt and can’t accept this. My heart is just too broken and I’m confused, conflicted and at a loss.

Mariella replies Get over it. These kids are young adults now and about to have a baby. Whatever your reservations were and no matter how justified your misgivings, the horse has well and truly bolted and your only option is to get behind your daughter and stepson and give them your support.

Life has its way of surprising us no matter how hard and fast we make our plans. With two children roughly the same age coming into close proximity the stage was set for bitter enmity, bored co-existence or forbidden passion. Yours have clearly opted for the latter and I’m afraid your rigid opposition and focus on your own sense of betrayal rather than their impulse to connect may be partly the cause. Who doesn’t want to rebel against their parents? And if you have ready access to a weapon of such potential outrage it’s pretty tempting to deploy it.

However justified, your rigid opposition may be partly the cause

The truth is I’ve no idea what it was about your domestic set-up that may have propelled them into each other’s arms, but it is clear that it’s too late to sit around stroking our Freudian beards and dissecting the antecedents of their union. The fact is that your kids have a baby on the way and are unlikely to part in the near future.

It may well be that further down the line the relationship proves itself to be the terrible mistake you foresee, but since you can’t prevent it, isn’t it preferable to ensure you’re there to catch them if they fall? Until that time, for harmony’s sake and to prevent irreparable damage, you need to rise above your sense of betrayal, stop taking their relationship as a personal affront and help your family to move forward, however dysfunctionally, to a shared rather than severed future.

I admit it’s not the most ideal of beginnings, but neither is dancing around a nightclub at 4am high on illegal substances, which is where quite a few of my now smug-married friends first discovered their passion. Romance works in mysterious ways and there really is no right way to encounter the person that potentially you could share a future with.

In many ways their youth and premature leap into parenting are far more concerning than the fact that they were raised alongside each other. Unrelated children raised together and falling in love in adulthood is not something we should encourage, but neither can we prevent it, and it’s one of the many less-explored complications of modern blended families. Forced physical proximity may lead to contempt or a deep sense of security, and depending on their circumstances one or other is bound to appeal.

I’m presuming your stepson was based at his mother’s, while your daughter lived with you. I appreciate this is a pretty old-fashioned assumption, but I suspect had the circumstances been different it would have been unusual enough for you to mention. That means that although they have known each other since childhood their sense of sibling connection may not be as honed as it would have been otherwise.

In days gone by, before the internet brought strangers from across the globe into your potential dating circle, it wasn’t at all unusual for cousins to marry, the blend of small catchment and regular contact proving an intoxicating one. Nowadays we think we know better about all of this stuff, but all too often our most basic instincts are the cause of covert shame rather than prompting frank and healthy discussion.

There’s no alchemy available that can take these children back in time, so accepting their relationship and showing them love and support at what must be a strange and stressful time would seem to be your primary duty as a parent. They may well live to regret their current course of action, but if you make yourself a stranger yours will be the greater loss by far.

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