Kat is a 27-year-old woman who slept with her sister's partner after a night of partying.

He says it was a mistake that will never repeat itself but Kat has fallen head of heels in love with him even though he's since proposed to her sister and is now engaged.

Read Kat's story below.

"My sister met Tumi four years ago and introduced him to our family a few months later. He was a great guy and I was happy she finally found a good partner, and so I welcomed him warmly into our family.

As their relationship progressed, Tumi and I became very close friends. It wasn't strange at all since he came around to my place with my sister very frequently. Also, my sister and I are really tight so me speaking to Tumi and laughing together didn't raise any eyebrows.

Besides, at the time I was also seeing someone who I was fully committed to, although admittedly, my relationship wasn't always perfect and I was slightly jealous of my sister and Tumi who seemed to never have fights or arguments.

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A few months ago my sister and Tumi invited me over to their apartment for some drinks and things got really wild to the point where my sister had completely passed out. Tumi and I who have always been sturdy drinkers continued taking shots into the night.

One thing led to another and Tumi and I ended up having drunk sex on the couch. And while I felt terrible, I also loved every minute of it.

The next day we went about things as normal except for a thick cloud of guilt that seemed to follow me everywhere. And I guess we silently agreed to keep what happened between us quiet for the sake of sparing my sister's feelings.

I had hoped I could just forget about it and pretend it never happened, but that's not how things have worked out for me because I think I've fallen for Tumi.

While we're definitely not as close as before and he rarely comes by, I can't stop thinking about him. When I do see him with my sister, I feel like dying. I'm so ashamed of myself but I can't help my feelings.

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After I'd had enough of my endless longing I made the decision to tell Tumi how I felt. To my complete disappointment, he seemed upset that I would even bring the matter up with him, and made it clear that it was a mistake that would never happen again.

I'm crushed and can't even believe I'm in this situation. I know that I am incredibly selfish for wanting him to drop her for me but the heart wants what it wants. Sometimes I even feel like telling my sister the truth so that they break up, but that would be heartless and honestly she doesn't deserve that."

We spoke to relationship expert Paula Quinsee and she has offered Kat the following advice.

"If Tumi has made it clear he does not want to anything to do with Kat, then she needs to respect his decision as well as the fact that he is now engaged to her sister. Kat and Tumi also need to be honest and transparent with Kat's sister as to what happened.

READ MORE: Forgiveness after infidelity: why women who stay deserve less criticism

This will help them start over on a clean slate and ensure that this situation does not come between them especially as they are embarking on a journey of ‘happily ever after’ - starting a journey together on shaky ground is not a good way to begin a future.

Kat needs to focus on her own life, picking up the pieces and moving on. In the beginning she may need to minimise her interactions with the sister and Tumi especially if they are in the middle of wedding planning as this will only cause more angst for her.

She needs to focus on her own life, interests and goals and how to go about achieving that versus comparing her life to that of her sister (and fiancé). She should also look at seeking professional help to help her work through her emotions and feelings."

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