Child Support for Illigitimate Children Living with Father

Hi, good day. My boyfriend and I where together abroad for 2 years. He had 2 kids when he was a minor they where not married but since then the woman and the kids lived in his parents house. My boyfriend and his parents are the ones providing for the kids. She is unemployed since then. We are planning to get married soon and we wanted to know what we can do to secure our rights as a married couple against the mother of the kids. We understand that it is compulsory for us to give and provide for the kids but what is not clear to us is that if it is necessary for us to give monetary to the mother despite that the kids are with his parents custody and is provided by him and his parents alone. Thank you.