Custody of US born child




My sister (US born citizen) married a Mexican man. After marriage, she spent 60% of her time in Mexico and 40% in the US. She never became a Mexican citizen since their plan was to move to the US. My sister became pregnant and had a child in the US. My nephew lived in the US until he was 4 months old. They all moved back to Mexico City and have been there for about 9 months. They have not started any paperwork to make my nephew a Mexican citizen. My sisters husband now wants my sister to move out of his house. They have not filed any divorce paperwork, but he is verbally aggresive towards her. He says he is going to hire a lawyer to get full custody of their child since he believes she is unfit to care for him (Since she does not have a job and has been a stay-at-home mother). My sister does not want to stay at their home anymore and wants to move back to California. She is unsure what to do because she does not want to leave my nephew and be pinned with child abandonement. She also doesn't want to just take him and bring him to the US becuase she doesn't want to be accused of kidnapping. My whole family is wondering who my sister needs to contact. Should she contact a Mexican lawyer or an American lawyer?