My brother's in California and I'm in New York. Can I get POA over him here and he sign papers in California?

Maybe if you both had the same bank? Also you do know that POA is only good till the person dies, then they have no more power. Durable Power of Attorney is for health stuff I believe, Anyway about your problem, I'd talk to your bank and see what they say. There's probably a way somehow I'm thinking.