My boyfriends has 2 sisters who have lived with their grandma for 8 years now because his mother gave up her parental rights, and just recently their grandma passed away in a horrible car accident. We are wondering how we can get custody of them?

He will have to file with the courts to request legal custody. Assuming there is no else requesting custody, uncontested, the courts are likely to grant his request vs. placing the children in foster care. This is of course assuming is able to provide the appropriate care.