Just if anyone cares -- I finally got a second opinion. I brought my younger one to the orthodonist where my older daughter went. It is now a bit out of our way, but let them know my situation -- I thought the local one -- in my current town quoted us too high.
He quoted $500 less. So about $5,600 vs. $6,100 and I might be able to squeeze out a sibling discount even though my older daughter is out of braces, she is still coming in for retainer checks so in theory she is still an active patient. I get final quote next week. Hopefully the xrays confirm and since I have had previous experience with this orthodonist and his quote for my older daughter was pretty much what were we ultimately charged, I can have some confidence.
Damn kids are expensive.