I have to agree with perpetuallysad on this one.

While I could go on about how it isn't likely to work out if they are really that different that doesn't mean that it won't. There are certainly enough people who can stand as examples of having very good relationships when it seems they have nothing in common.

Truth is that even if she "seemed" like a perfect match there are no guarantees. Love is a tricky business and you really can't ever be sure how things will turn out until you try. It might work, it might not, there really is only one way to know for sure.

As far as my advice to you, be there for your brother. I am sure you know that already and are doing just that. Really though, that is all you can do with these things. Be there and hope that he is happy, hope that things turn out well. If they do not then help him through the difficult time, if they do than stand next to him and share his happiness.