I also used to be underweight (I might still be idk), and I'm also currently trying to get stronk.
Worst part of being skinny is when you sit down you feel your bones. Other than that, I love being skinny and lightweight. I was lucky to have no-one criticize me for my weight. I thought people only bullied fat people for their weight when I was a kid until I heard my mom talked about being her bullied in school over it. Im sorry your family criticize you for it. Look on the bright side though, once you start exercising, it's really easy to get a 6 pack.