Congratulations! I would present it as a very happy event that you are excited about, since that is exactly what it is. When we told my DSS that he was going to be a big brother (he is almost 12- I had a baby a year ago and am due again in May), he was apprehensive the first time and nonchalant the second time. Both times we talked about the impact on his life, positive and negative. We wanted to make sure that he knew that he would still be loved just as much. I think it was easier this time because he had already been through it.
As for the ex-inlaws, set firm limits, ignore them if they are disapproving and don't let them bring you down. I would also wait to tell them, since it's not really their business anyway!
Ooops - I completely confused myself.