Why does my family act this way?

Okay so if i look objectively at myself, i know that i’m not attractive or not THAT smart, but at the same time i don’t think i‘m dumb either.

My mom, dad and my little sister (14) have this thing where they like to make jokes about how incompetent i am. They always make jokes about how i’m very dumb, how i’m useless and how i have no social life. But i think that most of these aren’t true, or at least i hope so? Sure, in 7th grade up until 10th grade i had no friends at all, but in 11th and 12th grade i made 5 really great friends and i hung out with a lot of them often during school, but also during summer vacation. I’ve told my parents over and over again that i have new friends, but they never take me seriously and still think that i’m a loner without any friends. I also live in the Netherlands and i finished the highest level of middle/high school education there, without having to repeat a single year during these 6 years. Yet my parents still always call me dumb. Okay sure, i didn’t always get the highest grade, but i still graduated smoothly and even got into my dream university and i still get looked down upon. My sister is objectively prettier and smarter than i am and she always jokes about how i get really low grades and makes jokes about how i had no friends before and how i was cringe. The way she talks to me is so condescending, it actually hurts me. Now that i graduated they celebrated for me and are acting like they knew i would graduate, even though just a few weeks before my dad told my mom that i am dumb and that i would definitely fail. I’ve told my parents and sister that what they joke about is very hurtful, but they just shrug it off. My sister also keeps telling me that i also call her dumb and ugly, but i have stopped doing that for a LONG time. Yeah sure maybe i said that when i was 12, but i’m 18 now and i’m obviously not gonna call her such things. When i ask her when i said such things, she can never answer and keeps saying that i DID say that.

I just want to understand why my family looks so down on me even though i‘m doing my best.