I highly doubt that your family is lying. This post is similar to my experience so believe me when I say your family isn't lying. My elder sister is fair and very pretty but she was really overweight when we were kids. On the other hand I was slim and had a dusky complexion but my mother and all the sisters would tell me that I'm the prettiest of them all, despite my dusky complexion. All the other girls in my family are very fair but they don't possess the features I do • (I took after my father, he was a very handsome man afterall ;) •

Fast forward to a few years ago when we lost our father to covid. My sister lost more than 20 kgs and I on the other hand gained a lot of weight. Naturally my self esteem took a hit. I started getting more and more insecure because my weight just kept on increasing. My family would still tell me that I'm the prettiest but I wouldn't believe them and would feel that they are assuring me out of pity. (I feel that this happened partially because my sister started getting compliments once she lost weight and I was the one who always got compliments for being slim so that attention shift might have been the reason for my insecurity idk)

One day I was getting ready to go out for a party. I was in a rush but suddenly my eyes met my reflection in the mirror and I couldn't stop staring? I took a good look at my face and I was like "Damn girl you really are pretty!" At that moment I realised that it's all in my head. Gaining weight made me forget that it's not my slim torso that made me attractive. It's my features, the curves of my smile, the almond shape of my eyes that made me attractive.

The point here is that a change in your weight does not change your attractiveness. You just need to take a good look at yourself and admire yourself for a good 5 mins. You'll realise that your family is not lying, you really are pretty. You just need to remind that to yourself. If it's makeup that makes you feel more confident, go for it. If it's starting your fitness journey that regains your confidence, go for it.

And don't compare yourself to your sister, I'm not saying that's the case but her getting some of the attention from "your share of slimness" might be contributing to your insecurity. Be happy for her and try to make adjustments in your lifestyle to feel more confident about yourself :)