Contact the US Department of Labor, Wage and Hour Division. Don't contact the labor board, they only get involved in Unionized workforces, or places attempting to organize a union.
Contact numbers for each state are here:
http://www.dol.gov/whd/america2.htm
What you have described is not an uncommon practice, even Walmart has been busted for it. You don't have to give your sister's name, although she should make the call.
They will ask about the pattern, the frequency, and the number of employees.
No employee deserves to be intimidated into working off the clock. She's in fear for her job, so she does it.