AITA for trying to set a boundary with my little sister after she opened my birthday present?

AITA for Telling My 13-Year-Old Sister to Stop Treating My Stuff Like It’s Hers?

A young woman is struggling to balance sisterly affection with setting firm boundaries after her younger half-sister repeatedly disrespects her belongings — and even opened one of her birthday gifts without permission.

I (23F) live between my mum’s and dad’s houses. All my siblings are half-siblings from my dad’s side, including my 13-year-old little sister. I love her dearly, but she’s been testing my patience lately with her lack of boundaries.

She’s obsessed with anything that smells nice or feels fancy — face creams, body sprays, perfumes, lip balms, you name it. She owns dozens of these things already. I once counted 37 different lip products she’s collected. Despite having more than enough of her own, she’s constantly asking to “borrow” mine, which usually ends with her using up nearly the entire product.

For example, one of my perfumes was almost gone because she’d spray it 20 times every time she used it. If I say no, she gets upset. When I bought a new Bath & Body Works body mist recently, I left it at my mum’s house. She saw it in the background of a FaceTime call and immediately said, “That looks like it smells good — can you bring it next time you come over?” I joked, “I think we’ve got enough perfumes at Dad’s,” and she actually got visibly upset.

“She Opened My Birthday Gift Before I Even Saw It”

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I usually try to brush things off — she’s still young, and siblings share, right? But this latest thing really upset me. It was my birthday on Tuesday, and I went to my dad’s to celebrate. When I sat down to open my gifts, I noticed one package had already been torn open, with the contents removed.

My sister admitted she had opened one of my brother’s gifts to me — a cute self-care set with hand cream, a nail file, cuticle oil, and a small nail clipper — because she “couldn’t wait to try it.” Half the hand cream was already gone, the nail file clearly used, and the cuticle oil bottle wasn’t even properly closed. I know it’s not the end of the world, but it felt so inconsiderate. It wasn’t hers to touch, and she didn’t even think to ask.

I told her directly, “You need to stop thinking all of my things are automatically yours too.” That’s when things escalated.

“Her mum has always told her that siblings share everything — but that philosophy doesn’t work when it means disrespecting someone’s boundaries.”

“Selfish” for Wanting Boundaries?

My sister got mad, saying I “never share” anymore and that I’m “purposely” leaving stuff at Mum’s house so she can’t use it. Her mum — my dad’s wife — called me “selfish” for “belittling” my sister when she was just “trying to be close” to me.

I don’t see how respecting my property makes me selfish. I’m always kind to her and happy to spend time together, but I’m tired of every gift, lotion, or perfume I own being treated like communal property. I’ve told her before that borrowing is fine, but using up my things without asking isn’t.

Now I’m being called harsh and dramatic for wanting my own space and belongings. Part of me wonders if I’m overreacting, but another part of me feels like if I don’t put my foot down now, she’ll never learn what respecting boundaries looks like.

“It’s not about perfume or hand cream — it’s about teaching her that other people’s things aren’t automatically hers.”

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Drawing the Line

I’m not trying to punish her — I just want to set clear boundaries. She’s still young enough to learn, and I don’t want to resent her for constant overstepping. But her mum keeps undermining me by telling her that “siblings share everything,” which only reinforces the problem.

So, AITA for telling my little sister to stop using my things and for being firm about keeping some of my stuff separate — even if it means upsetting her or being labeled “selfish” by her mum?

“I love her, but love doesn’t mean letting her treat my boundaries like suggestions.”

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💬 Typical Reader Reactions:

“NTA. She’s old enough to understand that other people’s property isn’t hers.”

“Her mum is enabling her behavior — not ‘teaching sharing.’”

“Set boundaries now, or she’ll walk all over you when she’s older.”

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🌱 Final Thoughts

Setting boundaries doesn’t make you cruel — it makes you healthy. Sharing should be a choice, not an obligation, especially when others repeatedly take advantage of your generosity. This sister isn’t selfish; she’s simply learning that saying “no” is an act of self-respect, not rejection.

What do you think?
Should she keep enforcing her boundaries, or ease up to keep the peace with her little sister and stepmom? Share your thoughts below 👇

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