Is it possible that I can file for guardianship over my two nieces who live out of the country?

I don't see how a United States Court can exercise jurisdiction over two children who are outside the country. It might be possible to bring them to the United States under a plan for adoption. I think your best course is to consult a lawyer in the locality where the children reside.

Whether you can become the guardian of your nieces depends on the laws of their country. But, in any event, you will not be able to bring them into the US for permanent residence as your wards. You might want to talk to an attorney in your nieces' country about adopting them.

You left out several significant facts. Are these children US citizens living abroad or are they foreign nationals. If they are US citizens, you can bring them here and then file to be made the guardian based on your brother's death. The mother will have to be notified and does have an automatic right to the children, but it may not be in their best interest to live with her if she has not been in their lives. You also don't say who has legal custody of them now. Your aunt? Anyone? Did your brother leave a will indicating who should be the guardian for his children? If so, the court would likely follow that intention. You also don't say how old the kids are. You are quite young. If the children are young (6 or under) the court may well consider that differently than they would placing older kids with someone your age. If the children are not US citizens (or have a green card) then the laws of the country of their citizenship would govern. Since you don't say where they live, I have no idea what the laws might be. Please seek a lawyer's advice as this is a complicated problem and you need the help of someone who knows what they're doing.