what are you going to do if someone/anyone requested you then you denied it?

well, if they want you to be a ninang then I guess they think good of you. No parent or grandparent for that matter would want their daughter/grand daughter to have a godparent that did not pass their standards. taking this in a more positive light, as a godparent, you have the chance to influence the child and the mother as well. try to take that into consideration. If you are still not comfortable with the idea when you can politely refuse and tell them that you do not feel that you are the right person to be a godparent to the child.