How do I tell this sweet boy my sister is just using him?

My little is 15 and im 25. Even though we have a large age gap i do still act as her stister and as a parent role at times.

she's always getting herself in trouble at school and is the stereotypical mean girl mainly because my parents have spoilt her rotten because she's the baby of the family.

She was dating a guy much too old for her in the sixth form at school. For those of you that aren't from the uk sixth is for teenagers/ young adults once they leave school and go on to further education from ages around 16 to 18. Some sixth forms are already built in to some high schools so the transition isn't so difficult especially in smaller towns.

So this guy she was dating was 18 and yes that may not sound to big of an age gap but what does a 18 year old who can go out drinking and clubbing in the uk want with a 15 year old its kinda creepy. He also wasn't a very nice boy and would constantly cheat on her, talk to her like dirt and would encourage her too sneak out at night.

Eventually my family band her from seeing him and took her phone away and made her go church with my grandma on Sundays hoping shed learn something and grandma would set her straight.

Things started to seem really good and like she was changing for the better. One weekend when I was down for bbq with my family and she was with this lovely boy called Charlie she'd met from church who shed also known from school.

Charlie is extremely kind, polite, intelligent and well kept. I got talking to him and hed said how happy my stister had made him and he seemed really smitten with her. I finally thought my stister had been saved.

After everyone left i heard my stister on the phone to her friends. Firstly I wanted to know how shed got her phone back so I listened into the conversation and I heard her calling Charlie all sorts of horrible names and basically saying she was using him to get back at her ex and make him jealous as Charlie gets bullied at school by her ex
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I dragged her out her room by her hair and gave her the ass whooping of her life and called her a "little whore" and other things I ain't proud of. She seemed in complete shock and started screaming and my dad had to break it up. I explained the situation and my parents seemed disappointed but did nothing. I dont wanna see this poor boys heart broken. Especially if he's being bullied already. He probably thought things were going good for him for once
How do I tell this sweet boy my sister is just using him?
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