I don't know what this is about and I don't know your age, and I really don't know your sisters age. But from what I can figure out you're about 17-24 and she's around the same age, and younger than you. Just me guessing here.
If her boyfriend is a thug who shoots people deals drugs and cheats on her/smacks her around, then that's serious.
But if he's just up to some minor things (I have no examples), something that doesn't really harm anyone directly then I say stay out of it and let life teach her a lesson.
But I don't know what this is about nor how old you two are so she might just get pissed off if you go lecturing her about her life choices. If she's old enough (23-24 and over) then let her make her own f**k ups and learn from them.
If she's a stupid little kid hanging with the wrong crowd then tell her what the f**k is up and how you feel about it.
People MUST make their own mistakes in life in order to grow, as long as they are not life threatening.
That's all I got, short and sweet.
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I'm not here to enjoy life, I'm here to withstand it.
AAA
Crosseyed God
:::)