Is it possible to sponsor two adult siblings who were refugees but have been resettled to Australia?

We would like to sponsor a brother and sister who are 23 and 18 years old to come and live with me and my husband here. They were refugees in Uganda but have been resettled by UNHCR to Australia almost a year ago. We were hoping for America but that didn’t happen. We want to become a family and give them back what they lost out on over the years. Would they apply for a visitor's visa or something different like a green card to work here and we would provide airfare and living accommodations, food and transportation. What would our chances be for this to happen especially when they are not blood relatives? We just love each other and want to be together. I spent 6 weeks with them in Africa about 2 1/2 years ago and I plan to go to Australia in January to see them again. Can you help us, is there any chance of us being together through our legal system?