Can an aunt get guardianship of her nieces and nephew who live in another state?

On what basis do you deem the parents unfit? Only a court can make that determination. If you think they are unfit and have some objective evidence, you can file a third party custody action in the state in which the children live. The court can then evaluate the evidence, likely appoint a guardian ad litem, and place the children with you, if the court deems placement with you in the children's best interest.