If the brother with downs, was just a fluke thing that happened to his parents, then your chances are no greater or no less for having a downs baby

However, if he carried the gene, which there are people that do carry the gene and pass that on to their baby, then your chances of a downs baby are a little higher.

A little inspiration for you I have a baby with downs, and then I had a perfectly typical baby right after her---my chances were higher, but it did not happen. Unlike mental illness, Down Sydrome is a genetic gene abnormality and not a hereditary gene. Just relax, I am sure that everything will be fine and your baby is who he/she is, regardless. If you have anymore questions, I will be happy to repsond, I have done a lot of research on this topic, espicially having a baby with ds and then getting pregnant again so soon after.
Julie