Is it normal for a parent to say "you're stupid" to their own child?

i have a little sister , she is elementary school.shes very creative and loves to write and her drawings are amazing.but shes a little bit slow with numbers.shes not dumb or anything but she struggles a little in math, but i think its nothing wrong , she just need a little practice and some support...im in high school i barely have time for my self but i try to help her...the story.we were having dinner with my parents and my dad started asking stuff about her cl and something about math, i didnt listen about the details but what i heard after that was an insult, he called my little sister stupid.let me rephrase "...you dont even know that simple -blabla...you go to school for god's sake!! how stupid can you be?"and i can see how hurt she was.that really broke my heart.and my mother didnt do anything but laugh!..this isnt the first time that has happened, it happened to me too when i was young and i cried alot but i try to just not listen to them..its my lil sis im worried abt..w