Mother dies, Father in prison. Who gets Gaurdianship/caregiver

Mother died 1/9/18. Father in prison. Couple were currently married. Father wants me (his Mother) to care for the children.. I have his oldest Son from a prior relationship and my Son wants all the kids to stay together. I actually had legal custody of the kids until they were given back to their parents on 7/30/17. The other Grandmother that came into town for the funeral took off with the girls to Florida (where she lives) She finally responded back to my calls and she said she has already obtained Gaurdianship in Florida without my Son's knowledge. ??? Is that even legal? And what can we do to get them home? I do not have much money so can't really afford an attny.