First of all, your story is very inspiring! I think I'd follow the other mamas' advice and just be happy with your one brother and your friends' responses, make it work the best you can with the help you get willingly.
And trust me, you may not really *know* their financial situations! I have a relative (through marriage) who presents as very successful, generous, giving, yet my husband and I discovered that he is really struggling greatly and is almost bankrupt. It's an accident that we know. If we hadn't found out, we would think he and his family were much more secure than ours.
The other thing to consider is that not all people feel comfortable adopting a family over one season, since it's temporary. My parents always donate to church, but never directly to the "adopt a family" group because they thought their money would go further if it was directed to a resource focused element (like ESL services, housing assistance, or jobs training). It was just their thing. My mom volunteers at the ESL office of her archdiocese, translates for the school district, offers homework help for kids, etc....
One other thing you can do is send out another email in a little while that says something like, "Thanks to so many of you for responding! Here are the details of what we're going to do. If you have any other suggestions or ideas, or questions, let me know. I hope to have any donations (fiancial or otherwise) by _______date_" That way they aren't on the hook for not previously responding, and they get a reminder in case they may have been interested but didn't respond.
Hope it all works out for you!