Im worried about my little brother .. what will i do?

Im worried about my 4 year old little brother. Sometimes my mum can be really mean to him and its becuase when he was born my mum went through an enormous amount of pain which she didnt have so much with me or my other brother. But sometimes she yells at him in the face and sometimes even hits him for no reason. One time he just walked in the room and my mum started yelling at him "no one wants you around here" and then smacked him right in the face. Sometimes she doesnt even let him wear his PJ's when he goes to bed he just sleeps completly naked. Other times she doesnt even wash his clothes which i find really sad for him when he hasnt done anything wrong. There was even a time when he told my mum that he needed new trainers becuase the ones he had were to tight for him then my mum yelled right in his face "do you think im ******* made of money, do you think i have money to buy you new trainers all the ******* time" but then he answered her back saying "but you spend money on Kyle (my other brother aged 12) and Lee (me; aged 14)". Then she wacked him right in the face with her bare hand and the red mark afterwards was terrible, i felt really bad for him and i have never heard him cry that loud in all his life. One time he even told me that he didnt want to be here and said "i dont know why she hits me and stuff i havnt done anything wrong", in which he hasnt. It isnt his fault that he was a tough child to have at birth, its not like he wanted to be here; so really my mum shouldnt take it out on him. But othertimes my mum can be really nice to him; she can watch a movie with him whild cuddling into him on the couch, she reads his stories, washes his clothes; kisses and cuddles him, holds his hand when there out and about and buy him things that he wants, but i dont understand why she be's mean to him and also be's nice to him on some occations. Im worried about this and wondering what i can do?