What to do about children abandoned with me?

My brother (whose wife is deceased) recently dropped off his 3 kids for a "visit" and is not coming back. For many reasons, this is better for them. However, I want guardianship to be legal. Should I contact a lawyer, or can I resolve this by simply talking to social services? Will I have to go to court? Will I have to become a foster parent (and my home a registered foster home) in order to keep them? Has anyone else had family members abandoned with them and if so, how did you resolve it? I want what is best for the kids and they need stability and security. Any good advice is appreciated.