My sister took her child from her husband?

My sister married and left the United States years and years ago. She is a dual citizen, so are her children. She recently took her youngest child and left her husband and came back to America. They are not divorced and she is going to file for a divorce here in the states. She did not have her husband/the child's father's permission to leave or to take her child with her. The child is a dual Citizen but I think it would constitute kidnapping. Is it?