How to deal with this nagging feeling

Soo me and my sister go to the same tuition and you know family obviously compares our marks and everything and i really feel jealous of my cousin sister or maybe just angry because shes like very rich and I’ve always been the smarter one but recently ive been slacking off she got more marks than me in a test im really disappointed in myself i sometimes used to like not really mean but yeah i used to show her my achievements in academics but now i think she just thinks im just dumb and feel really bad and i feel like i failed and i want study and do better but something in me is just dead. Pls help me how can i deal with this feeling i get into depression thinking about these thoughts late at night