'My sister crossed a line with my teenage daughter. I'll never forgive her.'

As told to Ann DeGrey.

When my sister Sandy* and I were growing up, she was a lot more extroverted than me. She was always loud and funny, and she loved being the centre of attention. As a teenager she'd sneak out of the house at night to meet up with her boyfriend and tell me all about it the next day.

As for me, I was the absolute opposite; very sensible, always doing the right thing. We were close in our own way, but completely different people.

As adults, we stayed friendly, but the differences in our personalities were even more obvious. I had my first child at 24 and threw myself into motherhood with everything I had. Sandy, on the other hand, said she never wanted kids.

She preferred to party, travel and be a free spirit. She always said motherhood wasn't for her.

But when my daughter Annie* was born, something about Sandy changed. She was besotted with her from the moment she saw her. She really loved being an aunt and just adored my daughter.

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At first, it was sweet. She'd show up with cute clothes and toys, take Annie for walks, call herself "Fun Aunt Sandy." Then, as she was growing up, she'd let her stay at her house, where she'd stay up late and eat too much sugar, but I didn't mind. That's what aunties are for, I told myself.

But things changed as Annie got older, especially once she hit her teens. Sandy started doing things behind my back. She took Annie to get her ears pierced when she was 13.

This was something I had said she needed to wait until she was sixteen or older to do. I found out when Annie came home proudly showing them off, and Sandy just laughed and said, "Come on, it's harmless. She was ready."

But I was furious. Not because of the piercings, but because I wasn't even asked. I felt completely undermined.

That wasn't the only time. A year later, Sandy let Annie drink alcohol at a family BBQ. Just one or two drinks, she said. "It's better she tries it around people she trusts," she told me when I confronted her. But that's not the point.

I'm her mother; it's for me to decide what she's allowed, not Sandy.

Still, Annie loves her, and Sandy loves the attention. They text constantly, share private jokes, go out for lunch, and have their own little world that I feel increasingly shut out of.

The final straw came when I found out Sandy had taken Annie to the doctor and signed as her guardian to get her on the pill.

Annie had recently started dating a boy at school, and instead of coming to me, she went to her aunt. I only found out because I found her pills and the prescription. I was horrified!

When I asked Annie why she didn't come to me, she said, "Because Aunt Sandy doesn't freak out. She actually listens."

That was really devastating for me. I sat in my room and cried after that conversation. It's not because I don't want Annie to be safe or informed, but because I want her to feel like she can trust me. I've done everything I can to be a good mum.

I've always been there for her, caring for her, helping with school projects, worrying about her — yet somehow, I still ended up the one she sees as uptight and annoying. Meanwhile, Sandy gets to rush in and be seen as the fun, understanding one.

I tried to talk to Sandy about it; the contraception, the alcohol, the decisions she keeps making without me.

She just rolled her eyes and said, "You're overreacting. Annie needs someone who gets it. You should be grateful she has me."

But I don't feel grateful. I feel robbed of a relationship I've worked so hard to build.

I know part of this is just teenage behaviour. It's normal to pull away from your parents and confide in someone else.

But what makes it harder is that the "someone else" is my own sister, and she keeps crossing the line between supportive and interfering.

I'm not trying to be the enemy. I just want my daughter to know she can come to me. That I won't always have the cool reaction, but I'll always have her back.

I've tried to have more open conversations, to listen instead of lectures, and to show her I'm here no matter what. But it's hard to rebuild trust when someone else has stepped into your place.

I love my sister. But I wish she'd remember that Annie isn't her daughter.

She doesn't have to deal with the illnesses, or the stress and anxiety she faces from time to time – she just gets the fun side of her.

And maybe one day, Sandy will understand that being the cool aunt is easy. Being the parent?

That takes a lot of hard work and much more than zero discipline and a few shopping trips.

*Names have been changed to protect privacy.

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