Keeping A Fast After Sehri
This is a decision you have to make yourself after considering all the releavnt facts related to your situation and with an utmost honesty. I can only help you in understanding of the principle underlying current situation. Brushing your teeth doesn’t have any effect on your fast, but if you were already drinking something then you can complete it even if Sehri time is due. If you were under an honest misunderstanding regarding sehri time and drank something under a misunderstanding of time, then this won’t effect your fast too. But if you knew that Sehri time is over and you drank something after that, then it’s better to repeat that fast at a later date.
Kaffara is only when one breaks their fast on purpose. I cannot comment on your situation since I do not know all the facts therefore, the final decision lies with you. However, to be on a safer side, there’s no harm in repeating the fast at a later point.