IOS – In a group text, how do you reply to the same numbers that were originally messaged

Your brother's Apple ID probably still has his phone number associated with it for iMessage. The smoothest way to deal with this is to have him disassociate his phone number from his Apple ID.

Apple has a self-service web site that explains how to accomplish this:

• https://selfsolve.apple.com/deregister-imessage

If that doesn't work, this older KB article has a helpful section "Unlink a phone number".

If he has other Apple devices (Mac or iPad) that are using his Apple ID, your iMessages may be being delivered there. If none of those devices are available when you send him an iMessage, your phone will eventually offer you the opportunity to resend the message as an SMS or MMS.

Unfortunately you can't disable iMessage on a per-contact basis.

The possible solution for this is turn your network off for a moment, and try sending it as an SMS, if it goes good, else hold the blue bubble for a second and tap on "Send as Text Message". I use to do this for iOS 5, I am not sure if this has updated for iOS 7 as I haven't tested till now.