Childrens allotment

Hi i want seek an advice regarding my sister's xboyfriend they had 2 children for like 10-12 yrs old.this man is working as a seaman and giving allotment for like 15k below for the 2 kids everymonth.my sister is now married and the husband was the one who taking care of the kids studies and whatever the kids needs and wants for they are all staying in malaysia now.now my sister is demanding for 30k everymonth for he always lies about the allotment he says 20k but everytime we check at d bank it always less than 15k sometimes 11k.for how many yrs my sister was just keeping quite.but now he is saying he will send 20k again for he will be working in other company.my sister insisted 30k.does she have anything to do bout this matter or laban for this?how and what should my sister do first.and by the way he is 3rd engineer as of the moment.