What to do when kids are being neglected?

my sis has this boyfriend that shes been with for a year and some change and he has three kids. Now my sis is black and has two kids of her own that are black and her boyfriend and kids are white. But this is not the issue my niece and nephew have told me that they don't want to go home they all are living in a three bedroom house. and my niece and nephew say that they have to fight for a place to sleep. and their mom does not take time with them anymore i did my nieces hair last week and all of her hair was broken off and dry. they have bite marks on them from bedbugs and lice. I've had them for the last week and they do not want to go back home. I should also say that even tho she's not taking care of her kids she is taking very good care of his three kids. what should I do? I know i have to take them back home but how can i knowing that their not being fed right and mistreated. Is there anything i can do with out calling children and youth.