Narc parents living vicariously through other sibling, but refusing to pay for my medical expenses

So as I've stated previously here I am 17, and I don't have the money to pay for the procedure I need to get and I've also said my mother has bad "finances" but the more I see her spending habits the less I believe that it has something to do with how much money is going in but how much is going out. I need an orthodontic procedure done and its very costly and every attempt at asking my mother to book appointments and get this appliance placed in she says "We just can't afford it." But here's the kicker, as I've been doing this all summer my mother spent TEN THOUSAND dollars on just a horse for my sister whom she vicariously lives through by legitimately forcing her to do horseshows, nobody wants the horses besides my mother, my mother also has spent thousands of other dollars on expenses for this crap this summer to all whilst never taking me to my appointments. My sister constantly expresses disinterest in the hobby and often expresses extreme distress because my mom berates her the whole time she's doing it, even when she's doing well my mom is so awful to her it makes me feel so horrible. I never got this treatment because I learned very early on that doing any hobby my mother wants me to do ends up stacking against me and the expenses to cover it and how I perform etc will be used over my head, so instead of doing it to me she does it to my sister and almost treats my needs apathetically like they don't matter. She never brings up that I need this procedure, never makes effort to take me to my pediatrician to get a sleep study set up to see if I need cpap, and if she does its only to act like she cares, its always a simple one off like "We need to go get your stuff done" and if I respond with a "Why don't we." Its a wall of excuses like "We just can't afford it." Or "Maybe next week we'll schedule." And when next week comes its back to "Well I just can't right now." And consider that this is all happening WHILE SHE IS SPENDING THOUSANDS ON MY SISTERS EXPENSIVE "HOBBY" But she won't put any money towards what I need because I don't do the song and dance for it. I'm so tired of this.