How do I convince my parents to buy me new clothes?

My mom is always buying my sister clothes for like 20 dollars per shirt everytine we go to the mall.she gets like 5 bags full of clothes. and jewelry the thing is she is always so bad and hurting me she doesn't make good grades and she's so mean and always yelling at my parents but still my parents get her whatever she wants.I'm just walking with them hoping to get just one shirt. But I don't even get anything at all but my sister does. I've been saving my clothes and wearing them for like 3 years.some shirts show my stomach and Its really embarrassing and I want to cry.please help me please try to convince my parents but they just yell at me I only get $2 and $6 shirts I feel so unmoved