I have two brothers who are 14 and 16 years younger than I am, and they look up to/respect me as a sort of parental figure in (maybe) a similar way to the situation that you have. I can imagine being asked to intervene in a similar situation. However, I have to say that while I was very close to/involved with them when they were little (under age 10), I haven't seen them nearly as much in the last few years, and have no idea what's going on in their lives in terms of friends/aspirations/developing values etc. They now live in a different state than me, and VERY far away, so I only see them about 1-2 times a year, and only long enough for a "fun" visit, not for in-depth relationship building. So all that is to say that I don't currently have the kind of relationship with them that I would need to have in order to say, "hey, I heard you and mom are having some problems. Can you tell me what's going on?" And expect them to actually answer. So: if I were in your situation, here's what I would do:
(After running it by your mom) I'd call your sister, and say, "Hey, I heard you and mom have been getting in each other's hair lately, and I thought it might be nice for you and me to spend some time together and give you guys a break from each other. How would you like to come spend the weekend at my house sometime in the next week or two?"
Ideally, she will say yes, and you guys can hang out and bond (do something she likes/wants to do, and something you like/want to do). Then maybe Sunday morning over breakfast, say, "I really wish I could help out with you and mom, but I don't know what's going on. If you think I CAN help, I'm here to listen and help you guys figure it out, but I'll leave that up to you. I just want you to know that I care about you, and I get a little worried when I hear you're not coming home after school, cause this isn't the safest place, and I don't want anybody taking advantage of you."
And then I'd drop it. Hopefully she will either continue the conversation, or bring it up with you later, or call you the next time she has a problem with your mom. Or maybe she will decide that you caring about her safety is enough of a reason to stop putting herself in danger. There are a number of ways she could react to this. The important thing, though, is to make sure that she feels like you're in her corner, and that you're looking out for her, and will listen - not that you're there as a disciplinarian, or that you'll condemn her/jump to conclusions. You want her to feel safe talking to you. Once she's willing to have a conversation, it shouldn't be too hard to figure out what's going on - and then you can help your mom understand. Because ultimately it is her job to be the parent, not yours.