Is punishing for accidents during potty training okay?

My little sister is 3, and my step-mother has been forcing her to potty train since she was no more than two months out of turning 2. For a while she was doing really good, but then she started having accidents and my step-mother would scream at her and spank her because "she's tired of cleaning up her messes." As if she didn't know that potty training came with messes. Well now, she's 3 and a 1/2 and is terrified to tell anyone when she (occasionally) has an accident because she knows if mommy finds out she'll be hit. 
As a result of that, potty training her has taken some major back steps. I guess, I'm just asking do you guys think negative reinforcement for accidents during potty training is okay?

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