If my stepson is being abused can I take him away?

My stepson lives with his mother in Indiana. His father, our 3yr old and myself live in New Mexico. It has come to our attention that his mother is taking care of three girls ages 3,5, and 7 and has a newborn of her own. My stepson is 7 and I think it is unappropriate for him to be living with girls not related to him so close in age. But what bothers me is that since she has all these other children to look after she is neglecting our son. He has to fix his own food, he is not in any sports. This probably isn't that serious but I think it would be better for him with us. We own our home both have jobs, he has his brother, he would get more attention. We don't have a court order in place for custody or visitation. Is it possible to just bring him here, and make her fight for custody to get him back?