Explain to her that she needs to be a kid and that her sister broke up with him because he was weird and unstable and she could not help him. That she can't help him either.

Also trying to get the parents to understand too will help. They need to know that there little girl is seeing a guy who's 19 and you don't feel he's good for her. You can act like the big sister/brother to her. She might appreciate it later on.

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I'll wait