Your ancestor's brother is not your ancestor, so he will not appear in any ancestor report of your ancestors.
You may find a descendants' report more useful to include the information you want to show.
If a report from your current database will pick up more people than you want to include, consider making a copy of your file and unlinking the unwanted from the wanted, or exporting a focus group, or using tags to export the wanted people.
I don't want to flood you with a whole lot of instructions here, so do take some time to look up some of the terms I've used in the Help file in Legacy, and come back here and ask more questions if you need to.
I've "talked" before about the unlinking bit, along with deleting them, too - see the Legacy User Group archives at firstname.lastname@example.org
/... for a post where I've described my use of it.
If that link doesn't work (I've had problems posting links to this archive in these boards before), go to http://www.legacyfamilytree.com/LegacyLists.asp
and scroll down to find the link to the archives. Then once there, search for a thread with the subject line "Gedcom file for a specific family surname only - I don't understand how to do it" and that will find it for you.
Hope this helps. :-)