Emotional abuse?

Long story shot my partner has custody of his 2 children with the mother having fortnightly visit over night. It has come to light that mum is telling my step daughter to come home and say she doesnt like being t our house because we are mean and she is also telling my step son he can go back and live with her. This is having a massive impact on both childrens behaviour the boy is lashing out at school and the girl is off her food at my wits ends because social services dont seem to think there is an issue