First, just because they married quick doesn't mean it won't last. It sounds like you have more issues with his quick marriage than he does. Maybe you're worried for him because of his previous bad marriages, or afraid that he is going to get burned again, and your nephew also hurt in the process. Maybe you have some unresolved emotions over your mother's passing, which is natural, and a horrible thing for a family to deal with.
However, it doesn't always mean that it was the wrong decision for them. Also remember that some people have religious beliefs or other similar views that would make them want to get married on a specific day, or combination of days (11/12/13 for example).
Second, is there anything about this woman that you don't like? Is she bad to your brother? His child? Was it her choice or his to leave you out of the loop?
If all of this is on your brother, I think you need to sit down with him and have a heart to heart about what is going on and what, if any, issues that there are between you two. Maybe something was missunderstood and he felt you wouldn't be supportive, or a million other things that could be misread during crazy couple of months.
If she is part of the problem as to what went wrong, then I think that the 3 of you need to have the dicussion. The most important thing is that you all clear the air and be honest about your feelings. Make sure you voice your concern about being left out of the equation, but also try to keep an open mind as to what they are saying.