As told to Ann DeGrey.

One afternoon Mum called my brother and I over to her house. She didn't tell us why she wanted to see us but, shortly after we arrived, she dropped a bombshell.

"I've got cancer and I might not be around for Christmas," she said. She told us the cancer was advanced, and her doctor didn't think she had long, maybe just a few more weeks.

My brother Adam* was devastated, crying right there at the table, hugging her. But Mum stayed calm, saying she'd had a good life, and she had made sure we were both in a financially good place so we didn't have to worry about anything.

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I remember thinking it was good that we'd recently done a DNA test which we'd sent off and were waiting on the results. It was something both Mum and I had wanted to do for ages just to see where our family really came from, as I'd always been told I looked like I had a Nordic background.

Mum and I found the whole process really fun and couldn't wait to get the results, which might show some distant cousins or funny ancestry surprises. But just weeks later, Mum was gone. The cancer had moved faster than anyone expected.

Adam and I were broken-hearted and couldn't believe we were organising her funeral. I couldn't imagine having to carry on with my life without Mum being there.

Then, two weeks after we buried her, an email arrived with our DNA test results. I asked Adam to come over to my apartment so we could look at it together. I was half expecting ethnicity pie charts or a note about Viking ancestors. Instead, we got something completely unexpected.

Adam's results came up first. The ancestry part looked normal enough, but when we looked at the relative matches, something was wrong. There was no connection to Dad's side of the family. None at all! Not even a distant link.

The report was clear: he shared plenty of DNA with me and Mum's relatives, but zero with the man who had raised him. We checked mine too. I matched Dad exactly as expected, but my brother did not. We sat there in silence, staring at the screen, reading the same lines over and over.

Maybe the lab mixed up the samples. But the more we looked, the clearer it became. Mum must have had an affair. My brother was definitely not Dad's biological son.

The dates lined up; he was conceived while Dad was away working on a big project overseas. I know this because Dad (who had passed away a few years ago) had once told me in confidence that he was always worried about how on earth they managed to conceive Adam when he'd only been home for such a short time during that project.

So we came to the realisation that Mum had been carrying a big secret.

I started ringing the older relatives one by one; Dad's sister, Mum's cousins, the aunt who was closest to Mum, and two of her old friends. I started each conversation by saying I was looking through old photos and wanted to hear some stories. Most of them were happy to talk.

Then, slowly, the pieces came together.

One uncle remembered Mum acting strange one summer in the late 1970s, going out more than usual, coming home flushed and quiet. Another cousin let slip that Mum had been friendly with a man from the local cricket club, someone Dad didn't know well.

A neighbour from back then mentioned seeing Mum's car parked at a café on the edge of town where she had no reason to be.

No one had hard proof, but the hints added up; she'd met someone. She had most likely fallen for him, at least for a while. And my brother was the result.

We still don't know who the man was. I've asked everyone I can think of, but the trail goes cold. Maybe he never even knew about the baby, or maybe he moved away.

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Or maybe Mum ended it the moment she found out she was pregnant and decided to keep the secret forever. Either way, she took it with her when she died, leaving us with a mystery.

Adam and I talk about it sometimes, but he doesn't seem overly bothered by it. He and Dad were never very close, so I feel that's one reason he's not as determined as I am to find out the truth.

But I keep wondering what Mum was thinking in those last weeks. Did she want us to find out? Maybe that was why she was so keen for the DNA tests? So, who knows if we'll ever solve the puzzle completely.

I'm going to keep plodding on with my amateur detective work in the hope I come up with some answers for my brother, as he has a right to know the truth.

*Names have been changes to protect identities.

Feature Image: Getty (Stock image for illustrative purposes).