I have noticed recently that my younger sister answers my parent's questions aggressively, How do I stop her from doing so?

How old is your sister? Maybe she's in a rebel phase. Try talking to her in a gentle way so that she can understand where you're coming from. Let her know the possible things that can backfire when she doesn't behave herself.

I think that it is best that you talk to her. For one, she might not be aware that she's behaving this way or she might have deep-seated emotions that's why she's behaving this way.