"I have a great aunt who is a sister of my paternal grandmother."

This is terminology that so many are confused about. Many folks call their aunts or uncles "Great" when they are actually "Grand".

The "sister" of your "Grandmother" is actually your "Grand Aunt". The sister of your great grandmother would be your Great Aunt. A minor point, but more accurate.

The "Relationship Calculator" in Family Tree Maker software (and other software) can help. Also see this web site:

http://stellar-one.com/genealogy/cousins.htm

I hope this helps.