Parenting my sister

So we lost my mom 2 years ago to a car accident. And my dad has never really been in the picture. My brother is now 20 and has custody over me and my sister since the accident happened. My sister is 14 and I'm 16 almost 17. My brother really doesn't watch over us and I take care of my sister. But I can't watch her 24/7. I know that she's been caught skipping school, and some other bad things. (And by that I mean very inappropriate things for anyone under 18) I've never tried to parent her. But she's getting involved with the wrong crowd and I don't want her to end up getting hurt, arrested, or God knows what else. I'm so worried about her and I don't know what to do, please help.