Splitting family files in Legacy

I have a large database of direct line ancestors as well as many extensions to the line, which contains over 12k individuals.

I did some research for one of my cousins, listing his wife's ancestors (which are all contained in my large database).  I have followed the instructions at Legacy's website for splitting his wife's database to a separate file.

Now, I would like to make a total copy of my large database for my cousin (which EXCLUDES his wife's ancestors for the exception of her parents.  However, in repeating the steps for splitting of a database, starting at my cousin's name, it results in a database of only about 900 individuals.  I suspect this is because it only copied his direct line of ancestors, and not the many extensions that are part of my large database.

My first guess at trying to accomplish what I want to do, is to simply go to my family file in the C directory and select to copy it, then open the copy and delete all of my cousin's wife's ancestors one my one.

Is my thought process correct?  Or, can someone perhaps tell me if there is an easier way to accomplish this task?

Thanks in advance to anyone that can assist me :)

Lori