Rights of a father to his illigitimate but acknowledged child.

My sister who is currently working abroad have 2 sons from a previous relationship, and the father who's now planning to go back in the Philippines is threatening her to take both children. they are not married and since been separated. The father does not give any financial support at all. My sister is afraid that she might lose her 2 sons since she can't go back to the Philippines right now, and doesn't know what to do. Can someone please enlighten us on what to do. I have several questions that needs some answer:
1) Can the father take my sister's children?
2) What are his rights even if he didn't support the children financially?
3) Can my sister keep her children even if she's working abroad?
4) What will i do as a guardian to stop the father from taking the children?
5) Must i file a case to pre-empt his plans? if so, what case am i gonna file?
My sister kept all the bank documents that proves that she's the one supporting the kids alone. PLEASE HELP US....

who has custody of the children now? whoever it is, resist any moves of the father until he files a case for custody.

you can also file a petition to be declared legal guardian of the children while the mother is away. then i recommend the mother has to come back and take care of the children