SInce this is for family, it is a slightly different situation. I would not charge my sister $450 for a cake for my niece :)
I thought you were talking about a random stranger. If she is paying though, you may want to note on her invoice a regular price and your discount that gets it down to the $175. And I would use the traditional serving amount (74) in the quote as well. This way if she passes along info to friends, you don't get a rep as the cheap cake lady giving it away. Show it costing $400 or whatever and you taking off 50% for the family discount. It'll make your sister appreciate the cake that much more and make sure others know it's worth.
And, to be honest- if it was my sister, I'd have no problem telling her that a sheet cake 6 layers high for 30-40 people is ridiculous!