Daughter Born out of Wedlock. Mother No longer Living. Please Help!

[Phoboes]

Hello, My name Is Phu. My father whom is a US citizen found out a couple years ago that he had a daughter born out of wedlock. My fathers child is between the ages of 35-40. Her mother did not wish for my father to know he had a daughter. After the mother passed his daughter searched for my father and eventually found him. My father was informed that he also has a grand daughter. Whom is now 18 years of age. Now that they have been united. I started to look for ways to bring my sister and niece over to the United States. Unfortunately I have read that the child (His Daughter) would need to have been legitimized before the age of 21 in order to get her a green card. I have researched as far as I am capable of to find a solution but have yet to find an answer. Until I stumbled upon this website.

So my question is would there still be a way to get my fathers daughter and grand daughter a green card to come to America?

Sincerely Phu Nguyen.