People keep calling me fat, what should i do?

Im 13 2/3 im in junors clothes. i wear a 10 if i want my pants to fit tight, a 11/12 if i want them to fit lose. in shirts i wear a L/XL and i have a almost flat stomach but i have C/D cup boobsand a med backside. and i have tried to diet but i can never keep up with it and my mom and dad wont help me because they feel i dont need a diet. but even my cousins say im fat like she would say i might weigh mor than she do and she is 34. she sayd im to big to wear a bikini. im tall also so i dont know it that helps yhu picture how big. i am 5"8'. i onli have strech marks on my hips right above my butt. can yhu tell me what i should do please