What would you do?

i picked my son up from his grans hse yesterday. my sister in law was their with her 3 yr old , mine s 2 , i knew he was crying and asked him why, he just pointed to his cousin ,so i understood then, she s always hitting or pinching him [this little girl is so spoiled ,what she wants she gets, she would play up for days untill she got it] we were all having a chat [the adults] when we heard the sound of a very hard slap the girl hit my boy across the face he turned blue with fright i nearly died i coaxed him for awhile and he went of playing again staying close beside me when she came up and done it again her mother never said a word i asked the girl why she done it she said she felt like it i was fit to KILL the little ***** at that stage i just left, i spoke to her mother a few weeks back about this and still she done nothing ,i think its wrong i would punish my kid if he done something like this , what s your view