Well, people dream all sorts of things, some of them have meaning and some of them don't. If she is praying for this and you know she is praying for this, it makes sense that you might dream about it. Possibly your subconscious is thinking about it even if your conscious mind isn't.
If her brother has expressed an interest in marrying you, and it is a reasonable possibility for you to marry him, you could consider it. It is a blessing to have an opportunity to marry someone compatible whom you might have some trust in. It could be nice to marry a friend's brother (although keep in mind it can also change the nature of your friendship, especially if there are marriage problems). Of course there is no guarantee that the marriage would be happy but it is worth considering if you think he might be a compatible spouse. However you should make your decision based on the real life circumstances around you, not just the dreams.
If he hasn't expressed an interest in getting married then there is no reason to give it thought - he may or may not be ready to get married right now.
Anyway, you are lucky to have a friend who likes you so much that she wants you to be family. :)