sponsor my sister to US; the case approved

[haprolife]

I sponsor my sister to US; the case approved. Her kids are older than 18 years old. Can they come? What is the age limit? Can my marry niece with kid come, too? Should I file for I-864, now? Try speed-up the process. How much do I need in the bank to sponsor them? 1sister and 2 kids older than 18 years old.
Thank you for your help. I’m appreciated.