My gut instinct was your boyfriend's sister is a witch! But then I had to question that....Is this something she does for a living? Is she giving up time from a paying customer to do your hair? And is that a discount rate? If the answer to these questions are all yes Then I think it is fair. If she doesn't do hair for a living, then I will stick to my first instinct.
It takes hours and hours to do the braids. This confident woman knows what her time is worth and is not shy to put a price on her skills. I admire that. Pay up with a smile or get a better bargain somewhere else.