My wife and I got married in Brazil, she is from Brazil, I am from the
US. We wanted to set up a pre-nuptual agreement before marriage, but
did not due to the extra month it would have required. We have been
living here for almost a year and have started to wonder if we could
change our brazilian marriage certificate, and how it holds up in the
US. We wanted to know the answers before we buy a house next year...
it would be our first large purchase together. She has a green card, I
am a US citizen.

Basically: 
1) how is a brazilian marriage certificate recognized in the US
2) is it possoble to re-marry in the US and get a pre-nuptual agreement
3) is it posible to change an existing brazilian marriage certificate

1.  A Brazilian Marriage will be recogized in the US unless there is
something very unusual about it.  Probably no problem there.

2.  Yes, but you don't need to.  You can make an agreement now if you
want without marrying again..

3.  You'd have to ask Brazil but you don't need to do so to make a
binding agreement now in the US.  I'm not sure about Brazilian law and
the effect of an agreement entered into in the US.

Just make sure to have a lawyer do it so it's done right.