@Alyseb630 I think it makes more sense that she hasn't acted until now.
She's still fairly new to the house and acting on every slight to her child (which, before this point, had been mostly harmless) could cause trouble in her marriage and/or status in the household. Especially, since she would be in conflict with the 'innocent & beloved' daughter, with basically no evidence.
Its also highly possible that she didn't percieve the bullying, or didn't consider it serious before this point. Remember that, in her last life, our mc didn't find out the truth until she was already being executed! Even in our society, parents often don't realize their child is being bullied until serious damage has already been done.