Yes, one pointy tooth is all it takes! lol

My take on the biting - she gets stimulated/aroused (not necessarily sexually) by the brushing, and it's natural to go bite your other cat in the neck.

Just as an experiment - give her a moment of cool down time after the brushing. Maybe some soft treats that you can give right after you finish brushing her.

See if that breaks the 'mood'.

On the other hand...she IS a Tortie!