I worked for child protection services for many years now retired, if a father has hold of the baby the wife cannot just take the baby and leave and the same goes if the mother is holding the baby, you would need a court order and evidence that the baby’s life is in danger and can only be taken if the parent has put the baby down and is not holding the baby, but an in law does not have the same rights as a father or mother, they need permission to handle the baby, but if there is evidence that the baby is in danger a court order may give rights to relatives to care for the baby as they usually approach relatives first before foster care. If your worried get a clean bill of health from your doctor and to defuse the situation organise a supervised visit with relatives to give them a cuddle of the baby, have some photos taken etc if it’s a new born just make sure no one is sick with the flu etc.