How to take a major step back?

So my little sister is starting to get with guys in her high school and I know how guys are at that age so I’m like kind of worried I don't know why. Like my older sister dated this guy and she told me later he was abusive and after they broke up he killed his current girlfriend. It scares me that she’ll get with a guy like that. But I don’t want to be like a parent when I’m not. She has a guy coming over today. I just need advice on how to not I guess worry so much
How to take a major step back?
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