My grandmother was pregnant with my youngest uncle when my dad graduated from high school...so you're not alone on this one!
I'm not sure why you think any of your or your husband's children won't be excited about the new baby, but, if they really are, I would simply tell them that love is infinite and that just because you love a baby doesn't mean you don't have love left to give them. Time, on the other hand, may be in short supply. I would let them know that a new baby requires lots of time and attention, the same time and attention THEY received when they were babies. I would include the visiting children in the love and care of the new baby whenever they're over. If you're all a family, then helping out is part of the deal.
When I was in jr. high school, my dad married a wonderful woman with three sons and a daughter. I was delighted to finally have brothers. I was not so delighted that I had chores when I went to visit because I was only there on weekends. After a few weeks of this, I let fly a sassy comment and my stepmom turned to me and said that I was part of her family, same as "her" children. If I was going to be a part of the family, then by gosh and by golly, I was going to BE a part of the family. I was floored because it was the EXACT same thing my mom always said to me! (I was sure all women went to mom school to learn these great lines...little did I know... <g>) After that, I had no problems with my "weekend" chores.
I hope you can do something similar with the new baby.
Good luck!