Immigration visa F-4

[Raja Ali]

My parents and two sister got immigration visa (F-4)
So they went to USA on 2013 and they came back after 6 month and due to health condition of my parents they didn't go back all of them
So now they are ok and wanted to go back
What will they do to get back ?
What papers they need?
Anyone guide me please?

[inadmissible]

Do they all still have their green cards? Are your sisters minors?