How can we have the custody of my nephews? when her mother abandon them for more than 2 yrs

My Nephew are 2yrs and 3 yrs old. We ( my mom and  sister) want to have the legal custody of this kid. We raise them, financially, emotionally and everything else that a mother should do. Before the kids was born, my brother ( kids father) and his girlfriend is been leaving in my house. I provide them food, lodging and whatever they needed. Right after she give birth to his 2nd child, she decided to go to  Malaysia and work. I said to myself, it's great she will help support the kids. But then she never send them anything nor call to talk to the kids. The kids know called my sister Mommy and they call me their Mama. I talk to the kids over the phone most of the time, in short I'm leaving out of the country. And i'm also the GOdMother of the 1st child if this will help and my sister is the Godmother of the 2nd child.

I'm afraid that 1 day, my brothers Girlfriend will show up and just take the kids, and we can not handle loosing them. We invested a lot for this kids, emotionally, financially even my sisters lovelife and personal life.

Is there anyway, we can keep the kids. and Legally have the custody of the kids?

Adopting them or just plain legal custody. I want the kids to stay with us, and have a great life.

Anyone here is a lawyer who wants to take our case. Or any Legal advice.

I don't know any law in the Philippines, regarding this kind of case.

Hindi mo nabanggit kung kasal ang magulang ng mga bata