Why does my sister always tell my mom everything?

Anonymous
I'm 19 and my sister is 11. I feel I can't trust her anymore. She always wind up telling my mom what I tell her to keep private. For example, I've been talking to this guy for almost six months. My sister has told my mom things she claimed she wouldn't tell. Like there was an issue where my sis claimed how she saw a flirty message on his phone. So I told the guy about it and he then sent me screenshots of the chats he received. I then sent them to my sister so she could point out the name of the girl from the message. It turns out my sis showed my mom the screenshots and it led to my mom yelling ag me and accusing me of stupid things like how I'm trying to "corrupt" my sister when I was just asking for a simple message. I regret and i don't think im gonna tell my sister anything anymore.
Why does my sister always tell my mom everything?
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