Look into resorts like Homestead, Nemacolin, Massanutten etc. They all probably hire summer help, the work will probably be waiting tables or leading activities (just got back from Masanutten and Homestead). The work itself may not be fun but the settings are beautiful and there seem to be lots of younger people working at the pool areas at least. And maybe this matters too, she will be in a mix of a lot of upper middle class/rich/educated vacationers. I can't promise they will be nice but she will see what edcuation can get you and show her a different lifestyle. Lots of folks strike up conversations with the folks working there and they may have helpful college/career advice. No matter what, you are spot on about getting your sister away and expanding her horizens before college.

If she is good with kids she could be a counselor at a sleep-away camp. Both my kids attend camp and their counselors literally changed my kid's lives for the better- amazing young kids that work hard for very little pay, but also get the experience of a lifetime.

The American Camp Association posts jobs for counselors:

http://www.acacamps.org/jobs

Also, have her check out this: https://www.raise.me/

It looks promising.

OP- are there any mentoring programs in her area? She would really benefit from having another adult in her life that had a fair outlook on life. There are lots if she is in this area.

if she's currently a junior and loves math or science, MIT has MITES: http://oeop.mit.edu/programs/mites/

Smith has several programs too and gives a fair amount of financial aid: http://www.smith.edu/summer/faq.php

Guest (dude) ranches out west hire high school students for the summer. A couple of links I found

http://www.coloradoranch.com/about-us/ranch-jobs
http://www.duderanch.org/employment.php

Deer valley YMCA camp in Somerset County, PA

OP - are you in the DC area? Your best bet might be having her live with you over the summer and get a camp counselor job here. That's what my sister did for a majority of her college summers...my brother-in-law too. My BIL actually got a hill internship! There are lots of opportunities here if she can stay with you.