How to deal with stuborn kids?

i have problem with my sis inlaws kids and my daughter..my sis lw's kids always hurt my daughter..my daughter is 2 years old and hers is 5 years old son hits her badly..they both are very stuborn her son always fights with her..and she is also getting his bad habits of hitting and pulling each others hairs..scrachting with nails , shouting and rolling on floor makes me mad.. i dunno how to keep my daughter and i get into trouble how to deal with their fights ..i dunno y but i have started disliking her son very badly..coz of his bad habits..and wat ever my daughter gets from hubby he wants same thing frm hubby ..i can't help it.... i want to make my daughter disciplined well mannered girl please help me.....