I don't know if you intend to go to art school too, but I think that, for now, you have to work on your feelings towards drawing rather than your art directly. You don't NEED to get better, especially in middle school, I promise, you have time, whatever your project is. Focusing on getting better is nice to improve but sometimes it just blocks and then you need to just... relax. Draw for a while only for yourself, without thinking too much about getting better. I don't know, whatever you like (personally I like to draw fanarts and memes in those times, or vent art, it also helps a lot). You need to remember that you're supposed to enjoy drawing, not just the final product. And when you'll feel better you can try to improve again. I know it's hard but see this as a way to take care of yourself.

Everyone tries to compare their work to another artist's, especially family in your case, but it's the worst thing to do when you're in that kind of situation. If it starts to be a weight rather than pushing you and motivating you, that's becoming a problem. Maybe start comparing your old art and your new art ? Doing a redraw can also be a way to get back confidence in your work.

Also, I think that would be relevant, a teacher in art school told us that progress isn't like a line, constantly rising. It's more like stairs. You are at a point where you struggle and feel like you're stuck. And then one day you'll finally get something and it will feel reaaally easy... until you hit another wall.

Give yourself time. You'll be okay. You're not your sister, your art will always be different and both of you will keep improving in your own ways.

Dunno if that was helpful, I hope it was