why is my mom so mean and strict?

my mom has 2 kids me and my little brother there is a 3 year diffrence between us.at his age I had to make all of the beds, do the dishes, wash the floors,weed the gardens and do laundry. now I have to do all that and make dinner take out the garbage, vacume and dust he dosen’t have to do any thing at all I don’t get an allowence I cant have friends over and if I don’t do something or I do it wrong im grounded for at least 3 days I didnt get a cell phone till this year and he got one a month before me I didnt get my laptop before him and I have to go to be at 930 and be up by 7 he takes his showers first so I only get cold water my parents say lifes not fair when I adress the problem he can go anywhere he wants I cant leave the front yard im 3 years older! I get 99s in all my classes because I pay atention in class and all I get is good job my brothe gets cs and when its report card time he gets a gift I never got that whats even worse is he hits me and puches me and is nasty to me but my parent don’t belive me they love him better why? I don’t have faimly anywhere else my grandparents are all gone and the rest of the faimly is out of state what do I do ? why do they do this ?