If my husband were to die before the children reached 18, who would get the children?

I married a man with two children. He has sole custody. His ex has had two younger children (not his children) removed from her care due to abuse and neglect issues. I've been raising her oldest two since they were 10 months and 2 years old. She is not active in their lives. Again her rights are terminated to her youngest two by the state. Is there anything my husband can do to prevent her from gaining custody upon his unexpected demise? Is there anything I can do to prevent them from having to go to her? I fear that if they had to go to their biological mom their welfare would be endangered.