I'd love to hear what you guys think.

According to the rules (as I understand them), a Part 91 owned and operated corporate aircraft can be used by any "sister" companies as well without a lease, and without being Part 135.

My question is this:

Does the primary holding company "bill" the sister companies for time used including flight crew, etc., or are they only allowed to bill DOC's?

I was asked for input, read the regs, and I wanted a second opinion. Your help is appreciated!