My sister in laws son was taken ... Need some help !?

Okay .. I need alittle advice .. Here is my story ... My future sister in law has 4 kids all have different daddy , her youngest son is 5 and just recently met his father ! However his dad has never paid any child support , Does not buy food or clothing for him . He has been visiting him on the weekend though and now today .He went and picked the little boy up from school and has taken off with him ! My sister in law was told there is nothing that she can do because she never went for ful custody of their son . The father will not let her see or talk to him ... he is on an Indian reservation right now with the little boy ... What can we do ? Are they right that she can do nothing ? How is this okay that the dad can just come take him when he has not taken care of him in 5 yrs ? Do you think its shady of him . It was not like she was a bad mom she is a good mom and she was not keeping him away from his dad so why did he do this ?