I am assuming that this is a network based device. In addition to the ports, you may also need to allow the executable or driver for the device.
I suggest checking with Brother support to see if they can provide information on what needs to be done to get this to work through the OneCare firewall or contact OneCare support for help.
How to reach support (FAQ) - http://social.microsoft.com/Forums/en-US/onecareinstallandactivate/thread/30400b52-7f26-4ba0-bc18-17e305329d90
-steve
Microsoft MVP Windows Live / Windows Live OneCare, Live Mesh, & MS Security Essentials Forums Moderator