Assuming she hasn’t attended yet (and therefore just took the LSAT, not the bar exam), most schools will accept letters of recommendation from anybody that can speak to your sister’s ability to succeed in law school. That can be her current employer, past employers, colleagues, past professors, community members she’s volunteered with, etc. If she can’t find literally anybody, she might be able to network with local attorneys to get a basic letter to help her meet requirements.
And law students complete internships whilst they’re in school, not sure who told her she couldn’t work for a firm during her studies. However, many schools do have a rule against allowing 1L (first-year) students to work as it could interfere with their studies. I agree that it sucks when you’re not already wealthy, though. She could see if the firm she currently works at would allow her to go on leave, coming back as an intern after her first year. Alternatively, she could do a part-time program.
Disclaimer: I’m also applying to law school this cycle, so actual lawyers may have better info.