How do i deal with his death?

You are not responsible for your brother's death. Infants that young can catch a bug from anywhere. It's winter, its cold, and he is new to his living conditions. This is alot for a young body to handle. I know that you are sad, but you didn't do anything wrong. Please help your parents, since they probably are very upset as well. You will get through this, but it may take some time. My condolences to you and your family.