It sounds to me like she was just messing around but to be perfectly honest, siblings will always be siblings, they will tease you for pretty much anything they can.

If you have a boyfriend, should it really matter what your sister says, she's probably a bit confused maybe. It doesn't matter what your sister says if you're happy with where you are right now, boyfriend or no boyfriend. It only matters what you think.

If you're still unhappy ask her why she thought what she did or even prove it to her.

If it's really bothered and upset you then, by all means, have a word with her about it. Try not to argue - that'll only get you more upset - but you could always ask her why she had that reaction. It might help you both understand eachother a little better and, you never know, she might appreciate that you actually asked her about it.

Try not to worry about it. Sisters, eh? As I said, you know what's going on and her choosing not to believe you is more her problem than yours.

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i think you're the same as me, we see things they'll never see