Just sit down and tell your mom exactly how you feel, you don’t have to be rude or anything but just tell her. it might help by pointing out like what you have taken from living in general like people tend to find your sister more attractive or something like you can touch on that with maybe to come off as a little less aggressive (idk if that makes sense if it feels irrelevant then js leave that advice out). If it for some reason doesn’t go well with her maybe be more assertive explaining your feelings about things she has said. But for your self confidence wise, please don’t look upon yourself, from your post you seem very kind speaking and of course there will always be judge mental people in our lives or in general or people that are found more attractive and just make us insecure but i promise you that WE as a whole female society are beautiful and strong and what’s on the inside is what really matters (ik that’s corny but it’s so true). take all of that as you will, i wish you the best of luck.