I need advice please ?

Before I tell my problem here is a little about me so you'll understand. I'm 14 years old. I just finished my first year in high school. I'm 5 feet and 1 inch. I also am 119 lbs. I always try to be like someone else especially my sister. It seems like everyone likes her better than me. I'm afraid to be myself. I think it's because i'm worried about what people think. I have low self esteem I think i'm too fat. One reason is because everyone in my house is smaller than me. I am the oldest kid in my house. Some of my siblings say i am fat. I feel like a talentless freak. My parents put me in a home schooling program cause they didn't want me to go to a high school. I never wanted to be home schooled. Ever since I've been home schooled I don't see my friends as much and I also gained 20 pounds. I want to the gym but my parents are super strict they barely let me do anything. I used to be good around other people. Now when I go to parties or sleepovers I'm glad to see my friends but I feel weird. I don't really talk because my sister tells me I always say dumb stuff. When I do talk I get sweaty and shakey. Then when I come home I think damn I should've said such and such. I don't even feel comfortable talking to my mom or aunts. The only person I feel comfortable talking to my sister who is 15 months younger than me. But most of the time she doesn't seem to understand me. My sister is very pretty she's also slim and tall she's like 5 inches taller than me. I'm short and fat I want to loose 20 lbs she said if I loose that much weight I will look bad. But i dont have a flat stomach. I would do almost anything for one. I want a therapist but I'm to shy to tell anyone, and I'm afraid that I'm going to look bad. I'm not really close to my mom or dad.