How about something simple like:
I wish to not be associated by name with members of my family known by the surname of Lastname.
I wouldn't worry too much about eloquence. You just want the judge to say; ok, you will hereby be known as Elf Middlename.
I actually do not see anything in the statutes requiring a reason to change your name or having to disclose said reason to the courts. I do not think they are too worried about why as long as there is something for a reason so the courts know it isn't for one of the reasons you cannot change your name.
http://www.in.gov/legislative/ic/cod.../ar28/ch2.html

