First, for Alsih2o -- I think that Djeta's point is that she didn't want to hijack this thread with a debate on the relative merits of the institution of marriage. I, for one, will let it suffice to say that SHE does not believe in it, but is not preaching this belief. That's enough for me.
As far as Redundant Silvermage's sister is concerned, I think everyone is pretty much already right-on with the advice. It's exceedingly hard to tell a fellow adult what is best for her. Djeta is right that the wedding day itself is magical in a different way than life afterwards can be. I do hope, however, that a 47 year old woman is mature enough not to be motivated by dreams of long white dresses and expensive floral arrangements.
If I may ask, is relgion an issue? If he were to adopt your nephew and/or any other children, would he really be comfortable not raising them to be muslims? That could be a scary and difficult issue for a couple already dealing with cultural and language barriers. Not an insurmountable issue, but a serious one nonetheless. It might be nice to talk about some of these things with your sister.
The other important thing is probably to make sure someone talks to your nephew about how this will change his life. He is a bit young to be part of the decision-making process, but he does need to have people talk to him and explain things to him.
I do commend you for approaching the situation with an open mind. That's very hard to do, especially if you had your doubts beforehand.