I am the ugly sister primarily because I don't conform to my parents' pretty rigid expectations for female beauty. My sister is blonde (by choice), dresses modestly, all tasteful accessories and classic clothes. I've never fit into that mold, and have always received a lot of criticism over my appearance from them, but especially my mom. I've literally been told "Why don't you try to look like your sister."
I've reached a point in life where it mostly doesn't bother me. I look the way I want and I have no desire to look how they want. But it definitely still impacts my confidence and feelings about my appearance at times. I had PPD after my daughter was born, and one way it manifested was through some pretty scary levels of self-loathing, especially around appearance, that I think must go back to growing up as the "ugly, nonconforming" sister. It was bad, and even involved some self-harm because I was so angry at my body and my appearance. It made me realize how deeply we are all impacted by the stories our families tell about us when we are young. I know my parents are wrong about me, I know I am not ugly. But I think some part of me will always believe otherwise. There's a little girl in me who just wants the love and approval of my parents and she'll never feel good, no matter how many times I tell her that her parents were wrong.
Hugs to you, OP. It's hard.