i was in the same situation a couple of years ago, i had my 7 year old son and was expecting my 2nd child, i did worry but there is not a lot you can do other than try and prepare them. it sounds like your doing the best thing anyway! my son loved his new brother and was brilliant! i hope you have the same outcome! im now expecting my 3rd child in a couple of weeks but am finding it difficult to prepare a 2 year old for a new brother!
good luck, kerry, 2 boys n blue bump!