Why is it that my sister thinks I am stupid in school whilst everyone at college and

School think I am clever? She thought I was clever till I got a grade 5 in maths whilst being in set 2, she also doubts me all the time and says if you fail one subject drop out of all of them because it's too much and too hard, stressing me out but I was achieving CCC in first round of exams and BCU in second round and was achieving As in English literature and Maths and C/U in Sociology but second round of mocks I got a B in Sociology and U in Maths. Why does she think I am stupid just because I can't balance my life with studies and I overdo things but I always achieve though? There may be circumstances affecting my work and she says it's too hard for you and teachers and students say I had potential and teachers know how good I am. How can my sister judge and tell me to keep dropping things when things get too much and drop out of BTEC if that's too much and do an apprenticeship or work in health as that requires minimal studying? And working is best for me?