As told to Ann DeGrey.

I remember the day I introduced my boyfriend Brad* to my younger sister, Ash*. She was a difficult person at the best of times, very judgemental. She didn't smile at him, just scowled as he shook her hand. I could tell right away she didn't approve of him.

Later, she cornered me in the kitchen, telling me she could already tell he was "arrogant and the controlling type." I just laughed and told her she was being ridiculous.

Ash was just 25 at the time, and she always had strong takes on my choices. I wouldn't say she was my best friend, but I loved her company and cared about her a lot.

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Brad and I had been together for a couple of months, and it felt really good. He was very strong, and I quite liked the way that he took control sometimes, in little ways. For example, I had a friend's 30th birthday party and was looking forward to it, but it happened to be on the same night as Brad's work event that he was counting on me to attend.

He told me he'd be very disappointed if I didn't turn up as his plus-one.

So, I decided to go with Brad and tell my friend I couldn't make her party due to a prior commitment. So, when I told Ash about this, she called it controlling, accusing him of cutting me off from friends.

"This is textbook toxic sh*t," she said.

That hit a nerve because my last boyfriend had been very controlling. But I assured Ash that Brad was nothing like my ex.

Still, she kept at it, firing off texts with articles on relationship red flags, zeroing in on controlling behaviours.

Whenever we caught up, Ash would sneak in jabs about whether Brad okayed my clothes or if he'd mind me heading out solo. Over the following months, I noticed a few things that worried me. Brad had his jealous side; he'd sulk if I hung too much with male coworkers.

One time, he saw my phone was unlocked, and I caught him scrolling through my messages. I had nothing to hide, but it bothered me a bit.

When I told Ash, she labelled it toxic and kept saying that I deserved trust and needed to get out of that relationship sooner rather than later.

She'd call late, checking in and hassling me about rethinking my future, warning it could worsen if I stayed with a guy like Brad. So the doubts started flooding in. Was he really that awful?

I had a few arguments with Brad. One time he was insisting I cancel an upcoming girls' trip. That was the first time I told Ash she might be right about the toxicity. Then things got worse and Ash really encouraged me to end things with him.

Brad was very upset and asked me to give him another chance, but I stood my ground. Ash was very supportive, telling me I needed to start over with a man who let me be myself. So, for a few weeks I felt free.

But then Ash started behaving secretively.

Then one night, after she'd had too much to drink, she caught an Uber to my place, and she was on the couch, scrolling through photos when she stopped on a photo that shocked me; it was a photo of her and Brad, naked in bed.

She realised I'd seen it and I tried to snatch her phone off her.

Now I think she did that deliberately let me see that photo. She really meant for me to see it, rubbing salt in my wounds.

I started to cry, asking her how she could hook up with Brad after telling me he was toxic?

She said it "just happened" and that Brad had contacted her to talk about me.

What I know now is that she'd totally orchestrated it, inflating his small issues to split us up.

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I feel she was driven by jealousy over my life and this was her power play, taking what was mine.

I contacted Brad, who claimed Ash had chased him hard, and he was sorry, but they are very much together. I believe him and don't really hold anything against him, as I'd broken up with him. But I'm devastated my sister stabbed me in the back like this.

She is still with Brad and I can't have anything to do with her anymore.

Our mum is pushing for us to make up, saying Ash is remorseful, but the betrayal was too deep.

The trust I had with my sister was ruined, and I've now pushed ahead with my life; I have a new boyfriend who is worlds away from Brad.

I feel I've recovered, but I still have times when I visualise that photo of her with Brad, and that calculated reveal.

Will I forgive her? Maybe. But it's not happening yet.

The author of this story is known to Mamamia but has chosen to remain anonymous for privacy reasons. The feature image used is a stock photo.

Feature Image: Getty.