I have to tell you, it sounds like you're carrying around some extra baggage. You need to remember that your parents' relationship isn't the same as yours. The reason for their divorce doesn't mean that you should fear the same thing happening in your relationship! And while it was obviously devastating to lose your brother, it doesn't mean that your babies will have to go through the same thing.
I would suggest that you try to focus on the best thing for your family now. Think about it from your husband's position, and try to be as objective as possible. It sounds like he might be confused himself, but why doesn't he want the 3rd, when it is clearly so important to you? Don't dismiss his logic, even if it seems irrational. His feelings are likely as strong to him as yours are to you!
Hopefully working it out together, the 2 of you can make the right decision for your family. And please remember that you have already been blessed with 2 children, so you're already a very lucky family!!! 
Good luck! Please let us know what happens and hang in there!!!!