File photo: A couple

Finding out that your partner has cheated is always devastating, but learning it was with a close friend or even relative is a whole other level of betrayal.

One woman has shared her heartbreak after her husband had an affair with her sister.

If that weren’t bad enough, she claims they had been struggling to conceive for years – but her sister became pregnant with the husband’s child, while they were still married.

The unnamed ex-wife revealed all in a letter to an agony aunt, explaining that her marriage ended shortly after they discovered her sister was expecting and her partner went off to help raise the baby.

Angry boyfriend and sad girlfriend at home. Couple worried after argument
He had an affair with her sister (stock photo) ( Image: Getty Images/iStockphoto)

She tells how he ended up marrying her sister – and the ex has had nothing to do with either of them since.

It’s now been five years and she claims her parents want her to meet her niece and attend a big family dinner.

But she’s torn over whether she can handle this.

Writing to Slate.com’s Dear Prudence advice column, that’s since gone viral online, she said: “When my ex and I were married, we had trouble conceiving and years of heartache. I thought our marriage was strong enough to survive this, then I discovered he was having an affair with my sister.

“We had a huge, traumatic confrontation and my then-husband and I decided to move and make a fresh start.

A few weeks after we moved, my sister gave the news that—surprise!—she was pregnant. My ex then divorced me to start a family with her.”

She continued to say that because she’d recently started a new job and they had a mortgage, it was “financially impossible” for her to leave, so she was forced to stay in a new city all alone after her husband left.

Close up of pregnant belly
Her husband had a baby with her sister (stock photo)

She went on to explain that for years she and her sister have taken turns attending family functions as she can’t bear to be in the same room as them.

Her parents were initially angry with her sister, but they reconciled when the baby was born.

After her parents asked her to attend Christmas with her sister this year, she finally decided to take a look at her sister’s Facebook page for the first time and what she saw made her “literally vomit.”

“There, I saw hundreds of happy pictures of them as a family,” she explained. “My ex-husband kissing her after she’d just given birth, photos of the happy first birthday party, family trips, etc.”

Despite the pain seeing this caused, she asked if it was time to grant her parents’ wish and try to tolerate their company.

Responding as Dear Prudence, agony aunt Emily Yoffe was incredibly sympathetic in response.

“It’s no wonder what you saw made you sick to your stomach. The violation against you was enormous,” she wrote.

“You have been in a bubble of denial for the past five years, so a tidal wave of evidence of the happy family life you feel these cheaters stole from you is bound to be overwhelming.

“If over the past few years you’d had some minimal contact with them and knew your niece slightly, you would be in a different place emotionally now.”

She urges the woman to give herself some time, potentially speak to a friend or therapist and remember that her niece is the “innocent party” in all of this.

She added that Christmas was too much of a “loaded time” to try the first meeting, and instead suggested picking a random weekend.