What rights does a grandma have over her granddaughters, how can she get custody of them?

the father of the little girls died a years ago(one is gonna be 2 years the other is 6 years), their mother went to jail she has a felony and is getting deported, so the little girls are staying with their aunt(the mom's sister), the grandma lives in mexico, but came for a couple of weeks to visit them and she wants to know how she can get their custody and take them with her if it is possible. What will happen to the little girls, we don't want the government to take them and put them in foster care.