Is it considered a sin to react emotionally to protect a younger sibling from domestic violence, and will a mother's supplication (du'a) against her children for illness or death be answered, knowing that she is ignorant of religious matters?

You were wrong to anger your mother, as it is obligatory to be gentle and well-mannered with parents. We hope that what happened on your part falls under the possibility of Allah's forgiveness, as He says: "Your Lord is most knowing of what is within yourselves. If you should be righteous, then indeed He is ever Forgiving to the often returning [to Him]." A mother's prayer against her children is religiously prohibited, and usually, a parent's prayer against their child in a moment of anger is not answered. We advise you to strive diligently in honoring your parents, apologize to your mother, gently strive to advise your parents, and seek help from those who hold influence with them.