I can't understand why you are allowing her to decide whether to brush them or not. At 5 and a half, you decide, this is a health issue that will affect her new teeth that are developing as well as her general health. If she needed antibiotics but decided she didn't need them, would you let her choose?
My kids are almost 7 and almost 6 and are allowed to brush their teeth themselves with an electric toothbrush in the morning with supervision, and I set a timer to ensure that they give them a proper clean.
In the evening, it is part of the the routine they have followed since babies to have their teeth cleaned by me or my husband before bed.
I may sound strong on this, but my husband and I both grew up without a strong emphasis on toothbrushing and we suffered the consequences of extractions, gaps etc.
My advise is to tell her to do it properly herself or you will do it for her - full stop - no argument. Then follow through with an appropriate punishment if she doesn't e.g loss of something she values e.g. toy removed or loss of tv time. There are other times when gentle persuasion and encouragement is is appropiate - e.g reward if she does it correctly, but you need to get of the fence and take this in hand.
Personally I think that 5 and a half is too young to expect her to brush them properly every time - at least if you do it at the end of the day, she will go to sleep with a clean mouth.
You could also try getting the dentist to have a talk to her.