Rule number one - Do not get involved in a relation's or even a sibling's working terms and conditions unless you are 100% sure they are being wronged and they have tried all avenues themselves to get it corrected as there is nothing worse for an employer than to have some busy body thinking they are right when they probably aren't.
Do not call NERA unless you have proof of wrongdoing.
An employer can quite easily withdraw employment of a staff member in the first 12 months and extremely easy in the first 3 months of probation. - And certainly I would have no hesitation withdrawing employment from someone if parents or uncles started to get involved as I employee the person themselves, NOT their family and assuming they are over 18, they are adults and should be able to fend for themselves.
Now to probably give you possible answers...
Who has calculated the 89 hours - your niece or is it written on a payslip? I ask because breaks are not paid in the workplace, so a 9-6 with a one hour lunch is an 8 hour day.
What age is your niece.
She should have a basic contract / terms of employment (should be given within 4 weeks of starting, but can be up to 8 weeks without the company getting any flak.
Certainly have a check on things, and get her to get terms and conditions - she can say its needed for grant application or because she will be renting and a landlord wants it, or she is looking for a student loan.
But just don't get involved directly, her job will disappear very quickly especially in todays employment market.